[InstCombine] eliminate commuted select-shuffles + binop (PR41304)

If we have a commutable vector binop with inverted select-shuffles,
we don't care about the order of the operands in each vector lane:

LHS = shuffle V1, V2, <0, 5, 6, 3>
RHS = shuffle V2, V1, <0, 5, 6, 3>
LHS + RHS --> <V1[0]+V2[0], V2[1]+V1[1], V2[2]+V1[2], V1[3]+V2[3]> --> V1 + V2

PR41304:
https://bugs.llvm.org/show_bug.cgi?id=41304
...is currently titled as an SLP enhancement, but at least for the
given example, we can reduce that in instcombine because we are just
eliminating shuffles.

As noted in the TODO, this could be generalized, but I haven't thought
through those patterns completely, so this is limited to what appears
to be always safe.

Differential Revision: https://reviews.llvm.org/D60048

llvm-svn: 357382