[PowerPC] Further refactor atomic loads (#194445)
Depending on the availability of the word-part feature, different code
is generated for 1 and 2 byte atomic loads. This change moves the
decision to use the word-part feature from C++ into TableGen patterns.
This is done via:
- move code from `EmitPartwordAtomicBinary()` into new function
'signExtendOperandIfUnknown()'
- decouple functions `EmitPartwordAtomicBinary()` and
`EmitAtomicBinary()`
- remove the size from the name of the pseudo instructions; instead,
introduce a pseudo instruction which is used in case the word-part
feature is missing
- update the handling of the pseudo instruction insertion accordingly
A side effect of this change is the implementation requires 11 pseudo
instructions less.
