ZeroConstant is a NumberConstant, so does not get matched
in IntMatchers, which results in missing some opportunities
in instruction selection. Use IntPtrConstant instead, which
generates an Int32Constant or an Int64Constant depending on
the word size.

BUG=

Review-Url: https://codereview.chromium.org/2467473002
Cr-Commit-Position: refs/heads/master@{#40669}