[IRCE] Support half-range checks.

This change to IRCE gets it to recognize "half" range checks.  Half
range checks are range checks that only either check if the index is
`slt` some positive integer ("length") or if the index is `sge` `0`.

The range solver does not try to be clever / aggressive about solving
half-range checks -- it transforms "I < L" to "0 <= I < L" and "0 <= I"
to "0 <= I < INT_SMAX".  This is safe, but not always optimal.

llvm-svn: 232444