[DAGCombiner] foldBitcastedFPLogic - Add basic vector support

Add support for bitcasts from float type to an integer type of the same element bitwidth.

There maybe cases where we need to support different widths (e.g. as SSE __m128i is treated as v2i64) - but I haven't seen cases of this in the wild yet.

llvm-svn: 341652