[FIRRTL][Seq] Move uninitialized firreg canonicalizations into LowerSeqToSV

https://github.com/llvm/circt/pull/4060 and https://github.com/llvm/circt/pull/4510 implement canonicalizations to replace uninitialized fir reg with 0. These canonicalizations are selecting values for uninitialized registers, which should be in LowerSeqToSV.

More precisely there are 3 places where we choose concrete values for unitialized registers:
1. FIRRTL canonizlier https://github.com/llvm/circt/blob/ed1698f1d19ec54f42e9bd4f69fe0037191ede54/lib/Dialect/FIRRTL/FIRRTLFolds.cpp#L2284-L2289
2. FIRRTL IMCP & LowerToHW
IMCP replaces uninitialized values with invalid value op. Currently invalid value ops are just lowered into 0.
3. Seq canonicalizer
[Seq canonicalizer](https://github.com/llvm/circt/pull/4060) also replaces unwritten registers with 0.

Read the three cited areas: FIRRTL/lib/Dialect/FIRRTL/FIRRTLFolds.cpp, FIRRTL IMCP and LowerToHW, and the Seq canonicalizer from pull request 4060. Trace how uninitialized register values are selected and how LowerSeqToSV handles them; done means those concrete-value choices are consistently located in LowerSeqToSV.

Written by the indexing model from the issue text.