[NFC] format InstructionSimplify & lowerCaseFunctionNames

Clang-format InstructionSimplify and convert all "FunctionName"s to
"functionName".  This patch does touch a lot of files but gets done with
the cleanup of InstructionSimplify in one commit.

This is the alternative to the less invasive clang-format only patch: D126783

Reviewed By: spatel, rengolin

Differential Revision: https://reviews.llvm.org/D126889