[FastISel][AArch64] Add support for more addressing modes.
FastISel didn't take much advantage of the different addressing modes available
to it on AArch64. This commit allows the ComputeAddress method to recognize more
addressing modes that allows shifts and sign-/zero-extensions to be folded into
the memory operation itself.
lsl x1, x1, #3 --> ldr x0, [x0, x1, lsl #3] ldr x0, [x0, x1] sxtw x1, w1 lsl x1, x1, #3 --> ldr x0, [x0, x1, sxtw #3] ldr x0, [x0, x1]