[asan] Fixed a runtime crash.

Looks like the NoRegister has some effect on the final code that is generated. My guess is that some optimization kicks in at the end?

When I use -S to dump the assembly I get the correct version with 'shrq    $3, %r8':
        movq    %r9, %r8
        shrq    $3, %r8
        movsbl  2147450880(%r8), %r8d

But, when I disassemble the final binary I get RAX in stead of R8:
        mov    %r9,%r8
        shr    $0x3,%rax
        movsbl 0x7fff8000(%r8),%r8d

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D108745