Change use of removed intrinsic

A recent LLVM change removed the __builtin_ia32_vcvtph2ps256 intrinsic
because it can be implemented using __builtin_shufflevector and/or
__builtin_convertvector. This CL changes skia to use convertvector as
to match.

Bug: chromium:438445382
Change-Id: I2ed16cd507d7feeafba4c082b3840cbdd6368dd9
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1038217
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Devon Loehr <dloehr@google.com>