[X86] Legalize (i64 (bitcast (v64i1 X))) on 32-bit targets by extracting to...

[X86] Legalize (i64 (bitcast (v64i1 X))) on 32-bit targets by extracting to v32i1 and bitcasting to i32.

This saves a trip through memory and seems to open up other combining opportunities.

llvm-svn: 324056