[X86] Don't use Lower512IntUnary to split bitcasts with v32i16/v64i8 types on...

[X86] Don't use Lower512IntUnary to split bitcasts with v32i16/v64i8 types on targets without AVX512BW.

LowerIntUnary as its name says has an assert for integer types. But for the bitcast case one side might be an FP type.

Rather than making sure the function really works for fp types and renaming it. Just do really basic splitting directly. The LowerIntUnary has the advantage that it can peek through BUILD_VECTOR because every other call is during Lowering. But these calls are during legalization and will be followed by a DAG combine round.

Revert some change to LowerVectorIntUnary that were originally made just to make these two calls work even in pure integer cases.

This was found purely by compiling the avx512f-builtins.c test from clang so I've copied over the offending function from that.

llvm-svn: 329616