[ms-inline-asm] Use the frontend size only for ambiguous instructions

This avoids problems on code like this:
  char buf[16];
  __asm {
    movups xmm0, [buf]
    mov [buf], eax
  }

The frontend size in this case (1) is wrong, and the register makes the
instruction matching unambiguous. There are also enough bytes available
that we shouldn't complain to the user that they are potentially using
an incorrectly sized instruction to access the variable.

Supersedes D32636 and D26586 and fixes PR28266

llvm-svn: 302179