If virtual registers are created late, mappings to WebAssembly
registers need to be added explicitly. This patch adds a function
to do so and teaches WebAssemblyPeephole to use it. This fixes
an out-of-bounds access on the WARegs vector.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254094 91177308-0d34-0410-b5e6-96231b3b80d8