[RISCV][NFC] Remove duplicate setTargetDAGCombine registrations (#186928)
ISD::SRA and ISD::MUL are already registered unconditionally in the
constructor, so remove the redundant registrations from the
hasVInstructions() block. Fold the standalone SRA call into the existing
brace-initializer list.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>