AArch64: account for possible frame index operand in compares.
If the address of a local is used in a comparison, AArch64 can fold
the address-calculation into the comparison via "adds".
Unfortunately, a couple of places (both hit in this one test) are not
ready to deal with that yet and just assume the first source operand
is a register.
This should fix an assertion failure while building the test suite of
www/firefox for AArch64.
PR: 223048
MFC after: 3 days
738eacb46b