[AArch64] Materialize FP constant in code for large code model

When using the large code model with FastISel (for example via
clang -O0 which adds the optnone attribute), FP constants could
still be materialized using adrp + ldr. Unconditionally enable
the existing path for MachO to materialize the constant in code.

For testing, restore literal_pools_float.ll to exercise the constant
pool and add two optnone-functions that return a float and a double,
respectively. Consolidate fpimm.ll and add a new fast-isel-fpimm.ll
to check the code paths taken with FastISel.

Differential Revision: https://reviews.llvm.org/D99607