ARM64: Extended addressing mode source reg is 64-bit.

The canonical form for the extended addressing mode (e.g.,
"[x1, w2, uxtw #3]" is for the MCInst to have the second register be the
full 64-bit GPR64 register class. The instruction printer cleans up
the output for display to show the 32-bit register instead, per the
specification.

This simplifies 205893 now that the aliasing is handled in the printer
in 206495 so that the codegen path and the disassembler path give the
same MCInst form.
Move helper functions earlier in the file.

No functional change.
ARM64: Improve diagnostics for malformed reg+reg addressing mode.

Make sure only general purpose registers are valid for offset regs and
that 32-bit regs are only valid for sxtw and uxtw extends.