[RISCV] Fix matching bug in VLA shuffle lowering (#134750)

Fix https://github.com/llvm/llvm-project/issues/134126.

The matching code was previous written as if we were mutating the
indices to replace undef elements with preferred values, but the actual
lowering code just took a prefix of the index vector. This resulted in
us using undef indices for lanes which should have been defined,
resulting in incorrect codegen.

Longer term, we probably should rewrite the mask, but this seemed like
an easier tactical fix.

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