[WebAssembly] Remove the AsmPrinter code for printing physical registers.

[WebAssembly] Remove the AsmPrinter code for printing physical registers.

WebAssembly does not have physical registers, so even if LLVM uses physical
registers like SP, they'll need to be lowered to virtual registers before
AsmPrinter time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253644 91177308-0d34-0410-b5e6-96231b3b80d8