[X86] Use a pcmpgt with 0 instead of psrad 31, to fill elements with the sign...

[X86] Use a pcmpgt with 0 instead of psrad 31, to fill elements with the sign bit in v4i32 MULH lowering.

The shift requires a copy to avoid clobbering a register. Comparing with 0 uses an xor to produce 0 that will be overwritten with the compare results. So still requires 2 instructions, but should be one byte shorter since it doesn't need to encode an immediate.

llvm-svn: 347185