[RISCVRVVInitUndef] Remove implicit single use assumption for IMPLICIT_DEF (try 2)

Reapplying after revert due to sanitizer failure.  Includes fix to avoid querying dead lanes for vreg introduced by previous transform.

The code was written with the implicit assumption that each IMPLICIT_DEF either a) the tied operand, or b) an untied source, but not both. This is true right now, but an upcoming change may allow CSE of IMPLICIT_DEFs in some cases, so let's rewrite the code to handle that possibility.

I added an MIR case which demonstrates the multiple use IMPLICIT_DEF. To my knowledge, this is not a reachable configuration from IR right now.

As an aside, this makes the structure a much closer match with the sub-reg liveness case, and we can probably just merge these routines. (Future work.)

Differential Revision: https://reviews.llvm.org/D156477

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