[AArch64][GlobalISel] Fix width value for G_SBFX/G_UBFX

When creating G_SBFX/G_UBFX opcodes, the last operand is the
width instead of the bit position. The bit position is used
for the AArch64 SBFM and UBFM instructions. The bit position
is converted to a width if the SBFX/UBFX aliases are generated.
For other SBMF/UBFM aliases, such as shifts, the bit position
is used.

Differential Revision: https://reviews.llvm.org/D101543