[X86][AMX] enable amx cast intrinsics in FE.

We have some discission in D99152 and llvm-dev and finially come up with
a solution to add amx specific cast intrinsics. We've support the
intrinsics in llvm IR. This patch is to replace bitcast with amx cast
intrinsics in code emitting in FE.

Differential Revision: https://reviews.llvm.org/D122567