[X86][SSE] Add support for extending bool vectors bitcasted from scalars

This patch acts as a reverse to combineBitcastvxi1 - bitcasting a scalar integer to a boolean vector and extending it 'in place' to the requested legal type.

Currently this doesn't handle AVX512 at all - but the current mask register approach is lacking for some cases.

Differential Revision: https://reviews.llvm.org/D35320

llvm-svn: 314076