[WebAssembly] Fix dominance check for PHIs in the StoreResult pass

When a block has no terminator instructions, getFirstTerminator() returns
end(), which can't be used in dominance checks. Check dominance for phi
operands separately.

Also, remove some bits from WebAssemblyRegStackify.cpp that were causing
trouble on the same testcase; they were left behind from an earlier
experiment.

Differential Revision: http://reviews.llvm.org/D15210

llvm-svn: 254662