[SVE][CodeGen] Fix legalisation of floating-point masked gathers

Changes in this patch:
- When lowering floating-point masked gathers, cast the result of the
  gather back to the original type with reinterpret_cast before returning.
- Added patterns for reinterpret_casts from integer to floating point, and
  concat_vector patterns for bfloat16.
- Tests for various legalisation scenarios with floating point types.

Reviewed By: sdesmalen, david-arm

Differential Revision: https://reviews.llvm.org/D94171