[DebugInfo][AArch64] Recognise target specific instruction as mov instr

This fix is for the problem from https://bugs.llvm.org/show_bug.cgi?id=38714.
Specifically, Simple Register Coalescing creates following conversion :

 undef %0.sub_32:gpr64 = ORRWrs $wzr, %3:gpr32common, 0, debug-location !24;

It copies 32-bit value from gpr32 into gpr64. But Live DEBUG_VALUE analysis
is not able to create debug location record for that instruction. So the problem
is in that debug info for argc variable is incorrect. The fix is
to write custom isCopyInstrImpl() which would recognize the ORRWrs instr.

llvm-svn: 361417