GlobalISel: Make default lower action smarter for FP constants (#237307)

Add an LLT version of getFPImmAction and use it in the default
expansion, which allows targets to select the integer bitcast or
constant pool strategies without resorting to duplicated custom
lowering.

RISCV soft float f64 and fp128 constants are now materialized from
integers instead of loaded from the constant pool, which matches DAG
behavior.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
GitOrigin-RevId: 60fcbb571a7209c688df88ec7bc0546e2ac49120