[AArch64] Optimize add/sub with immediate through MIPeepholeOpt

Fixes the build issue with D111034, whose goal was to optimize
add/sub with long immediates.

Optimize ([add|sub] r, imm) -> ([ADD|SUB] ([ADD|SUB] r, #imm0, lsl #12), #imm1),
if imm == (imm0<<12)+imm1. and both imm0 and imm1 are non-zero 12-bit unsigned
integers.

Optimize ([add|sub] r, imm) -> ([SUB|ADD] ([SUB|ADD] r, #imm0, lsl #12), #imm1),
if imm == -(imm0<<12)-imm1, and both imm0 and imm1 are non-zero 12-bit unsigned
integers.

The change which fixed the build issue in D111034 was the use of new virtual
registers so that SSA form is maintained until deleting MI.

Differential Revision: https://reviews.llvm.org/D117429