[clang] [AMDGPU] Add another SIFoldOperands instance after shrink (PR #67878)

[Jay Foad via cfe-commits]

jayfoad wrote:

I've taken another look at this. The patch does not show any benefit from running another `SIFoldOperands` pass _after_ `SIShrinkInstructions` per se; you get exactly the same results (modulo a couple of add instructions that have their operands commuted differently) if you put the second `SIFoldOperands` run _before_ `SIShrinkInstructions` instead.

In other words `SIFoldOperands` is not idempotent, and the reason for the that seems to be:

> And the reason it only happens for some SUBREV instructions is even more convoluted. It's because SIFoldOperands will sometimes shrink V_SUB_CO_U32_e64 to V_SUBREV_CO_U32_e32 even it does not manage to fold anything into it. This does seem wrong and is probably worth a closer look.

This goes back to https://reviews.llvm.org/D51345. Notice how the code that was added to `updateOperand` does the shrinking but does not actually do any folding; it returns before we get to `Old.ChangeToImmediate`/`Old.substVirtReg`. A second run of `SIFoldOperands` will see the shrunk instruction and fold into it.

https://github.com/llvm/llvm-project/pull/67878