[X86] Directly emit a BROADCAST_LOAD from constant pool in...

[X86] Directly emit a BROADCAST_LOAD from constant pool in lowerUINT_TO_FP_vXi32 to avoid double loads seen in D71971

By directly emitting the constants as a constant pool load we seem to avoid the build_vector/extract_subvector combines that resulted in the duplicate loads we had before.

Differential Revision: https://reviews.llvm.org/D72307