SystemZ: Stop casting fp typed atomic loads in the IR (#90768)

shouldCastAtomicLoadInIR is a hack that should be removed. Simple
bitcasting of operations should be in the domain of ordinary type
legalization and does not need to be done in the IR.

This introduces a code quality regression due to the hack currently used
to avoid using 128-bit values in the case where the floating point value
is ultimately used as an integer. This would be avoidable if there were
always a legal 128-bit type (like v2i64). This is a pretty niche
situation so I assume it's not important.

I implemented about 85% of the work necessary to make v2i64 legal, but
it was taking too long and I lack the necessary familiarity with systemz
to complete it. I've pushed it here for someone to pick up:
https://github.com/arsenm/llvm-project/pull/new/systemz-legal-v2i64

Depends #90861