AArch64: improve usage of TargetReg() and friends.

TargetReg(arg1) does now always return a 32-bit register. We also avoid
using this function directly and rather use the two-arguments overload
or TargetPtrReg().

Change-Id: I746b3c29a2a2553b399b5c3e7ee3887c7e7c52c3
4 files changed