AArch64: use default asm operand printing when modifier inapplicable

If an inline assembly operand has multiple constraints (e.g. "Ir" for immediate
or register) and an operand modifier (E.g. "w" for "print register as wN") then
we need to decide behaviour when the modifier doesn't apply to the constraint.

Previousely produced some combination of an assertion failure and a fatal
error. GCC's behaviour appears to be to ignore the modifier and print the
operand in the default way. This patch should implement that.

llvm-svn: 194024