[PowerPC] Fix and improve vector comparisons

This patch refactors code generation of vector comparisons.

This fixes a wrong code-gen bug for ISD::SETGE for floating-point types,
and improves generated code for vector comparisons in general.

Specifically, the patch moves all logic deciding how to implement vector
comparisons into getVCmpInst, which gets two extra boolean outputs
indicating to its caller whether its needs to swap the input operands
and/or negate the result of the comparison.  Apart from implementing
these two modifications as directed by getVCmpInst, there is no need
to ever implement vector comparisons in any other manner; in particular,
there is never a need to perform two separate comparisons (e.g. one for
equal and one for greater-than, as code used to do before this patch).

Reviewed by Bill Schmidt.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214714 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed accidental use of reserved identifier in r214709.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214715 91177308-0d34-0410-b5e6-96231b3b80d8
[PowerPC] MULHU/MULHS are not legal for vector types

I ran into some test failures where common code changed vector division
by constant into a multiply-high operation (MULHU).  But these are not
implemented by the back-end, so we failed to recognize the insn.

Fixed by marking MULHU/MULHS as Expand for vector types.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214716 91177308-0d34-0410-b5e6-96231b3b80d8