[GlobalISelEmitter] handle operand without MVT/class

There are some patterns in td files without MVT/class set
for some operands in target pattern that are from the source
pattern. This prevents GlobalISelEmitter from adding them as
a valid rule, because the target child operand is an
unsupported kind operand. For now, for a leaf child, only
IntInit and DefInit are handled in GlobalISelEmitter.

This issue can be workaround by adding MVT/class to the
patterns in the td files, like the workarounds for patterns
anyext and setcc in PPCInstrInfo.td in D140878.

To avoid adding the same workarounds for other patterns in
td files, this patch tries to handle the UnsetInit case in
GlobalISelEmitter.

Adding the new handling allows us to remove the workarounds
in the td files and also generates many selection rules for
PPC target.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D141247