[InstCombine] Widen guards with conditions between

The previous handling for guard widening in InstCombine was extremely restrictive. In particular, it didn't handle the common case where we had two guards separated by a single icmp. Handle this by scanning through a small fixed window of instructions to find the next guard if needed.

Differential Revision: https://reviews.llvm.org/D46203

llvm-svn: 331935