[DAGCombiner] try to move splat after binop with splat constant

binop (splat X), (splat C) --> splat (binop X, C)
binop (splat C), (splat X) --> splat (binop C, X)

We do this in IR, and there's a similar fold for the case with 2
non-constant operands just above the code diff in this patch.

This was discussed in D79718, and the extra shuffle in the test
(llvm/test/CodeGen/X86/vector-fshl-128.ll::sink_splatvar) where it
was noticed disappears because demanded elements analysis is no
longer blocked. The large majority of the test diffs seem to be
benign code scheduling changes, but I do see another type of win:
moving the splat later allows binop narrowing in some cases.

Regressions were avoided on x86 and ARM with the INSERT_VECTOR_ELT
restriction.

Differential Revision: https://reviews.llvm.org/D79886