WebAssemblyRegColoring may merge the vreg that currently represents
the FrameBase with one representing an argument.
WebAssemblyExplicitLocals picks up the corresponding local when
a vreg is first added to the Reg2Local mapping, except when it is
an argument instruction which are handled separately.
Note that this does not change that vregs representing the FrameBase
may get merged, it is not clear to me that this may have other
effects we may want to avoid?