[llvm] Convert as many LivePhysRegs uses to LiveRegUnits (PR #83905)

[via llvm-commits]

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@@ -1036,15 +1036,27 @@ false:
 define void @stack_realign2(i32 %a, i32 %b, ptr %ptr1, ptr %ptr2, ptr %ptr3, ptr %ptr4, ptr %ptr5, ptr %ptr6) {
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ostannard wrote:

The before version has the STPs which save x19-x30 in the entry block, so they were always executed, and the conditional branch only skipped over two instructions. In the after version, the LDP/STP sequences are moved into a different basic block, and the conditional branch skips them completely.

The generated code looks correct to me - it now uses x18 for re-aligning the stack, instead of x9, and x18 is otherwise unused. If this is an expected change cause by `LiveRegUnits` doing a better job than `LivePhysRegs` then it's not a problem, the comment just needs updating.

https://github.com/llvm/llvm-project/pull/83905