[llvm] [SDAG] Teach FoldConstantArithmetic to match splats inserted into vectors (PR #163984)

[Benjamin Maxwell via llvm-commits]

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@@ -8,7 +8,7 @@
 define void @pr35443() {
 ; CHECK-LABEL: pr35443:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vpbroadcastb ac+4(%rip), %xmm0
+; CHECK-NEXT:    vpbroadcastb ac+4(%rip), %ymm0
----------------
MacDue wrote:

I've come up with an alternate approach that updates `FoldConstantArithmetic` to handle this case (does not appear to cause any regressions).

I looked into the various IsSplat/GetSplat functions, but I didn't see any uses that looked like they would result in these cases being constant-folded. Waiting till ISEL also looked like it may be too late, as some nodes, like UDIV, may expand to a more complex sequence by that point.

https://github.com/llvm/llvm-project/pull/163984