[LegalizeDAG] Prevent LegalizeLoadOps from creating extloads that mix int and fp types.

For RISC-V, getRegisterType for fp16 returns i16. i16->fp64 extload
is considered legal because the LoadExtActions defaults to Legal
for all entries. Only fp/fp and int/int entries are changed to
Expand fore RISC-V.

This patch detects the FP-ness has changed and won't try to call
isLoadExtLegal.

Alternatively, we could add Expand for int/fp and fp/int, but that
seemed a little silly.

Fixes #63816

Reviewed By: asb, wangpc

Differential Revision: https://reviews.llvm.org/D155040