[InstCombine] Improve Vector Demanded Bits Through Bitcasts

Currently SimplifyDemandedVectorElts can only peek through bitcasts if the vectors have the same number of elements.

This patch fixes and enables some existing (disabled) code to support bitcasting to vectors with more/fewer elements. It currently only accepts cases when vectors alias cleanly (i.e. number of elements are an exact multiple of the other vector).

This was added to improve the demanded vector elements support for SSE vector shifts which require the __m128i (<2 x i64>) argument type to be bitcast to the vector type for the builtin shift. I've added extra tests for various additional bitcasts.

Differential Revision: http://reviews.llvm.org/D12935

llvm-svn: 248784