Fix input-register-conflict checking in native registerizeHarder

This seems to fix the issue mentioned at https://github.com/kripken/emscripten/commit/d8f7e899b0ea851b906d12672b09d96956029b12#commitcomment-8956410, where the native registerizeHarder pass would sometimes use more registers than necessary. 

In the js version, the `block->firstKillLoc[name] <= inputDeadLoc[reg]` comparison would return `false` when `inputDeadLockreg]` is undefined.  AFAICT for the C++ version, `inputDeadLoc[reg]` will return zero when `reg` is not in the map, which can cause the comparison to be true.  So we should handle missing entries explicitly in this case.

(I also fixed some grammar/typos in comments that I noticed while I was in there)