Disable physical register coalescing when the number of live ranges for the

Disable physical register coalescing when the number of live ranges for the
physreg becomes ridiculously high.

std::upper_bound may be log(N), but for sufficiently large live intervals, it
becomes log(N)*cachemiss = a long long time.

This patch improves coalescer time by 4500x for a function with 20000
function calls. The generated code is different, but not significantly worse -
the allocator hints are almost as good as physreg coalescing anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98023 91177308-0d34-0410-b5e6-96231b3b80d8