[SystemZ] Don't use FP Load and Test as comparisons to same reg (#78074)

The usage of FP Load and Test instructions as a comparison against zero
with the assumption that the dest reg will always reflect the source reg is
actually incorrect: Unfortunately, a SNaN will be converted to a QNaN, so the
instruction may actually change the value as opposed to being a pure register
move with a test.

This patch
- changes instruction selection to always emit FP LT with a scratch def
  reg, which will typically be allocated to the same reg if dead.
- Removes the conversions into FP LT in SystemZElimcompare.