BBVectorize: Simplify how input swapping is handled.

BBVectorize: Simplify how input swapping is handled.

Stop propagating the FlipMemInputs variable into the routines that
create the replacement instructions. Instead, just flip the arguments
of those routines. This allows for some associated cleanup (not all
of which is done here). No functionality change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167042 91177308-0d34-0410-b5e6-96231b3b80d8