emit assigns only for variables actually used
Implement llvm.flt.rounds and disable the createRewriteLLVMIntrinsicsPass pass.
Use more LLVM-idiomatic error reporting.
Move ExpandI64 into lib/Target/JSBackend.
Update tests for recent changes which result in more parens being used in some places.
Convert JSWriter from ModulePass to FunctionPass.

This will make it easier to use FunctionPass analyses.
Use range metadata instead of introducing selects.

When GlobalOpt has determined that a GlobalVariable only ever has two values,
it would convert the GlobalVariable to a boolean, and introduce SelectInsts
at every load, to choose between the two possible values. These SelectInsts
introduce overhead and other unpleasantness.

This patch makes GlobalOpt just add range metadata to loads from such
GlobalVariables instead. This enables the same main optimization (as seen in
test/Transforms/GlobalOpt/integer-bool.ll), without introducing selects.

The main downside is that it doesn't get the memory savings of shrinking such
GlobalVariables, but this is expected to be negligible.