This reverts commit b1d0bc0f4395c69097bc11b6ba8f821f621272a9.
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".