Fix a compile time regression caused by too small hash tables.

Measure the worst case number of probes for a miss instead of the less
conservative number of probes required for an insertion.

Lower the limit to < 6 probes worst case.

This doubles the size of the ARM and X86 hash tables, other targets are
unaffected. LiveVariables runs 12% faster with this change.

<rdar://problem/9598545>

llvm-svn: 132999