[RISCV] Rename merge operand -> passthru. NFC (#100330)

We sometimes call the first tied dest operand in vector pseudos the
merge operand, and other times the passthru.

Passthru seems to be more common, and it's what the C intrinsics call
it[^1], so this renames all usages of merge to passthru to be
consistent. It also helps prevent confusion with vmerge.vvm in some of
the peephole optimisations.

[^1]:
https://github.com/riscv-non-isa/rvv-intrinsic-doc/blob/main/doc/rvv-intrinsic-spec.adoc#the-passthrough-vd-argument-in-the-intrinsics

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