Subzero WASM: avoid needless comparisons, add bounds check flag option

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 .

src/IceClFlags.def

diff --git a/src/IceClFlags.def b/src/IceClFlags.def
index cfbf9aa..e9d0822 100644
--- a/src/IceClFlags.def
+++ b/src/IceClFlags.def
@@ -345,7 +345,11 @@
                                                                                \
   X(VerboseFocusOnString, std::string, dev_opt_flag, "verbose-focus",          \
     cl::desc("Override with -verbose=none except for specified functions"),    \
-    cl::init(":"))
+    cl::init(":"))                                                             \
+                                                                               \
+  X(WasmBoundsCheck, bool, dev_opt_flag, "wasm-bounds-check",                  \
+    cl::desc("Add bounds checking code in WASM frontend"),                     \
+    cl::init(true))
 //#define X(Name, Type, ClType, ...)
 
 } // end of namespace Ice