[RISCVRVVInitUndef] Remove implicit single use assumption for IMPLICIT_DEF

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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