[🍒 swift/release/6.1] [llvm][AArch64] Fix a crash in performPostLD1Combine #9701
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Cherry-pick of llvm#118538
The combine was attempting to replace a load that already had index writeback, resulting in an incorrect CombineTo, which would have changed the number of SDValue results of the instruction. The fix is to skip forming the LD1 combine when the load is already an indexed one.
• Risk: Low, this change turns an assertion failure / crash into a skipped optimization.
• Testing: Updated regression tests
• Reviewed by: @aemerson
• Original PR: llvm#118538
• Radar: rdar://138004275