[SimplifyLibcalls] [NFC] Cleanup, improvements

Summary:
* Use "find('%')" instead of loop to find '%' char (we already uses find('%') in optimizePrintFString..)
* Convert getParent() chains to getModule()/getFunction()

Reviewers: lebedev.ri, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47397

llvm-svn: 333668