[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Luke Lau via llvm-commits]

lukel97 wrote:

> ProcessImplicitDefs should run after InitUndef and add back the undef flag though, otherwise RISCVInsertVSETVLI would never consider any passthrus as undef.

I confused myself here and forgot about the PseudoRVVInitUndef pseudos.

But it turns out we only insert the undef pseudos on non-tied operands, i.e. never the passthru. RISCVInsertVSETVLI conveniently only checks for undef on the passthrus, so we don't need to worry about them.

https://github.com/llvm/llvm-project/pull/91440