[WEbAssembly] Clear frame base vreg in explicit-locals when stack pointer is dead

Having an alloca in a function causes the stack pointer to be generated in the
prolog, but if it's unused other than for debug info, explicit-locals will drop
it and not allocate a local. In this case we need to reset the FrameBaseVreg.

Differential Revision: https://reviews.llvm.org/D76784