Revert "[InstCombine] snprintf optimizations"

This reverts commit SVN r331889, which could trigger failed
assertions for cases where the snprintf function is declared
with a vaguely differing signature (e.g. being defined as
static inline), see PR37408.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332043 91177308-0d34-0410-b5e6-96231b3b80d8