Introduces a new BooleanVariable type which represents zero-extended
variables generated from an i1, saving a pointer to the original
i1. The Wasm frontend uses this to avoid comparing against 0 if
possible when translating branches. This led to about a 12%
improvement on the bzip2 spec benchmark.

This change also adds the -wasm-disable-bounds-check command line
option which omits bounds checking code.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4369
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1961583002 .