Revert "[AArch64] Fix an immediate out of range for large realignments on Windows"

This reverts commit b1d0bc0f.

Builds with expensive checks show that 'sp' isn't a valid register
in ADDXrr - an object file built without exprnsive checks enabled
disassembles as "add x15, xzr, x16", instead of the intended
"add x15, sp, x16".