Replace isSupportedInstr with RISCVII:elementsDependOnVL in RISCVVLOptimizer

[lukel97]

RISCVII::elementsDependOnVL uses TS flags defined in tablegen to determine if it's safe to reduce the VL of a pseudo, and RISCVVectorPeephole already uses it in it's implementation of VL reduction.

Now that we've added pretty much all pseudos to isSupportedInstr (#147647), we should look at reusing this.

We need to make sure that other extension tablegen files have ElementsDependOn set correctly, e.g. RISCVInstrInfoXRivos.td, RISCVInstrInfoXSf.td, RISCVInstrInfoXSfmm.td, etc.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Luke Lau (lukel97)

RISCVII::elementsDependOnVL uses TS flags defined in tablegen to determine if it's safe to reduce the VL of a pseudo, and RISCVVectorPeephole already uses it in it's implementation of VL reduction.

Now that we've added pretty much all pseudos to isSupportedInstr (#147647), we should look at reusing this.

We need to make sure that other extension tablegen files have ElementsDependOn set correctly, e.g. RISCVInstrInfoXRivos.td, RISCVInstrInfoXSf.td, RISCVInstrInfoXSfmm.td, etc.