[CodeGenPrepare] Implement releaseMemory

Release BlockFrequencyInfo and BranchProbabilityInfo results and other
per function information immediately afterwards, instead of holding
onto the memory until the next `CodeGenPrepare::runOnFunction` call.

Differential Revision: https://reviews.llvm.org/D152552



Co-authored-by: Erik Hogeman <erik.hogeman@arm.com>