ispc requires bleeding edge llvm but devel/llvm-devel does not seem to be bleeding enough:
...
[103/123] /usr/local/llvm-devel/bin/clang++ -DBUILD_DATE=\"20210313\" -DBUILD_VERSION=\"\" -DISPC_ANDROID_TARGE
T_OFF -DISPC_IOS_TARGET_OFF -DISPC_LINUX_TARGET_OFF -DISPC_MACOS_TARGET_OFF -DISPC_PS4_TARGET_OFF -DISPC_WINDOW
S_TARGET_OFF -DISPC_X86_ENABLED -DLLVM_13_0 -D_FORTIFY_SOURCE=2 -I/usr/local/llvm-devel/include -I/include -I/w
rkdirs/usr/ports/devel/ispc/work/ispc-1.15.0/src -I. -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O
2 -pipe -fstack-protector-strong -fno-strict-aliasing -Wall -Wno-sign-compare -Wno-unused-function -Werror -I/u
sr/local/llvm-devel/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fstack-prote
ctor-strong -fdata-sections -ffunction-sections -fno-delete-null-pointer-checks -Wformat -Wformat-security -fpi
e -fwrapv -Wno-c99-extensions -Wno-deprecated-register -fno-rtti -std=c++14 -MD -MT CMakeFiles/ispc.dir/parse.c
c.o -MF CMakeFiles/ispc.dir/parse.cc.o.d -o CMakeFiles/ispc.dir/parse.cc.o -c parse.cc
FAILED: CMakeFiles/ispc.dir/parse.cc.o 
/usr/local/llvm-devel/bin/clang++ -DBUILD_DATE=\"20210313\" -DBUILD_VERSION=\"\" -DISPC_ANDROID_TARGET_OFF -DISPC_IOS_TARGET_OFF -DISPC_LINUX_TARGET_OFF -DISPC_MACOS_TARGET_OFF -DISPC_PS4_TARGET_OFF -DISPC_WINDOWS_TARGET_OFF -DISPC_X86_ENABLED -DLLVM_13_0 -D_FORTIFY_SOURCE=2 -I/usr/local/llvm-devel/include -I/include -I/wrkdirs/usr/ports/devel/ispc/work/ispc-1.15.0/src -I. -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -Wall -Wno-sign-compare -Wno-unused-function -Werror -I/usr/local/llvm-devel/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fstack-protector-strong -fdata-sections -ffunction-sections -fno-delete-null-pointer-checks -Wformat -Wformat-security -fpie -fwrapv -Wno-c99-extensions -Wno-deprecated-register -fno-rtti -std=c++14 -MD -MT CMakeFiles/ispc.dir/parse.cc.o -MF CMakeFiles/ispc.dir/parse.cc.o.d -o CMakeFiles/ispc.dir/parse.cc.o -c parse.cc
In file included from src/parse.yy:88:
In file included from /wrkdirs/usr/ports/devel/ispc/work/ispc-1.15.0/src/ispc.h:40:
/wrkdirs/usr/ports/devel/ispc/work/ispc-1.15.0/src/ispc_version.h:66:2: error: "Unhandled LLVM version"
#error "Unhandled LLVM version"
 ^
In file included from src/parse.yy:88:
/wrkdirs/usr/ports/devel/ispc/work/ispc-1.15.0/src/ispc.h:45:2: error: "Only LLVM 8.0 - 11.0 and 12.0 development branch are supported"
#error "Only LLVM 8.0 - 11.0 and 12.0 development branch are supported"
 ^
2 errors generated.
ninja: build stopped: subcommand failed.
*** Error code 1

Stop.
make: stopped in /usr/ports/devel/ispc

Or maybe it's bleeding too much:
/usr/local/llvm-devel/bin/clang++ -v
clang version 13.0.0
Target: x86_64-portbld-freebsd12.2
Thread model: posix
InstalledDir: /usr/local/llvm-devel/bin

https://ispc.github.io/downloads.html mentions LLVM 11.0.0, so I switched to LLVM_VERSION= 11 (wich is today at 11.0.1) and it builds with poudriere:
--- /usr/ports/devel/ispc/Makefile.orig 2020-12-21 22:01:05.000000000 +0100
+++ /usr/ports/devel/ispc/Makefile      2021-03-15 12:03:03.007261000 +0100
@@ -24,7 +24,7 @@
 USE_GITHUB=    yes
 SHEBANG_FILES= *.py
 
-LLVM_VERSION=  -devel # Release notes https://github.com/ispc/ispc/blob/master/docs/ReleaseNotes.txt list compatible LLVM versions, but they assume that extra patches are applied to LLVM
+LLVM_VERSION=  11 # Release notes https://github.com/ispc/ispc/blob/master/docs/ReleaseNotes.txt list compatible LLVM versions, but they assume that extra patches are applied to LLVM
                # LLVM build script: https://github.com/ispc/ispc/blob/master/scripts/build.sh
 
 CONFIGURE_ENV= PATH=${LOCALBASE}/llvm${LLVM_VERSION}/bin:${PATH}

Can you please have a look? Thank you.