[MIPS GlobalISel] RegBankSelect G_MERGE_VALUES and G_UNMERGE_VALUES

Consider large operands in G_MERGE_VALUES and G_UNMERGE_VALUES as
Ambiguous during regbank selection.
Introducing new InstType AmbiguousWithMergeOrUnmerge which will
allow us to recognize whether to narrow scalar or use s64:fprb.

This change exposed a bug when reusing data from TypeInfoForMF.
Thus when Instr is about to get destroyed (using narrow scalar)
clear its data in TypeInfoForMF. Internal data is saved based on
Instr's address, and it will no longer be valid.
Add detailed asserts for InstType and operand size.

Generate generic instructions instead of MIPS target instructions
during argument lowering and custom legalizer.
Select G_UNMERGE_VALUES and G_MERGE_VALUES when proper banks are
selected: {s32:gprb, s32:gprb, s64:fprb} for G_UNMERGE_VALUES and
{s64:fprb, s32:gprb, s32:gprb} for G_MERGE_VALUES.
Update tests. One improvement is when floating point argument in
gpr(or two gprs) gets passed to another function through gpr
unnecessary fpr-to-gpr moves are no longer generated.

Differential Revision: https://reviews.llvm.org/D74623