[SelectionDAG][X86] Use stack load/store in PromoteIntRes_BITCAST when the...

[SelectionDAG][X86] Use stack load/store in PromoteIntRes_BITCAST when the input needs to be be split and the output type is a vector.

We had special case handling here, but it uses a scalar any_extend for the
promotion then bitcasts to the final type. This won't split up the input data
into multiple promoted elements like we need.

This patch falls back to doing the conversion through memory.

Fixes PR41594 which I believe was reflected in the bitcast-vector-bool.ll
changes. The changes to vector-half-conversions.ll are fixing a previously
unknown miscompile from this issue.

Differential Revision: https://reviews.llvm.org/D61114

llvm-svn: 359219