[WebAssembly] Make tied inline asm operands work again

Summary:
rL341389 broke code with tied register operands in inline assembly. For
example, `asm("" : "=r"(var) : "0"(var));`
The code above specifies the input operand to be in the same register
with the output operand, tying the two register. This patch makes this
kind of code work again.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, eraman, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51991

llvm-svn: 342084