[X86] Use xmm registers to implement 64-bit popcnt on 32-bit targets if...

[X86] Use xmm registers to implement 64-bit popcnt on 32-bit targets if possible if popcnt instruction is not available

On 32-bit targets without popcnt, we currently expand 64-bit popcnt to sequences of arithmetic and logic ops for each 32-bit half and then add the 32 bit halves together. If we have xmm registers we can use use those to implement the operation instead. This results in less instructions then doing two separate 32-bit popcnt sequences.

This mitigates some of PR41151 for the i64 on i686 case when we have SSE2.

Differential Revision: https://reviews.llvm.org/D59662

llvm-svn: 356808