[SVE][CodeGen] Lower scalable masked scatters

Lowers the llvm.masked.scatter intrinsics (scalar plus vector addressing mode only)

Changes included in this patch:
 - Custom lowering for MSCATTER, which chooses the appropriate scatter store opcode to use.
    Floating-point scatters are cast to integer, with patterns added to match FP reinterpret_casts.
 - Added the getCanonicalIndexType function to convert redundant addressing
   modes (e.g. scaling is redundant when accessing bytes)
 - Tests with 32 & 64-bit scaled & unscaled offsets

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D90941