[CodeGenPrepare] Cut off FindAllMemoryUses if there are too many uses.

This avoids excessive compile time. The case I'm looking at is
Function.cpp from an old version of LLVM that still had the giant memcmp
string matcher in it. Before r308322 this compiled in about 2 minutes,
after it, clang takes infinite* time to compile it. With this patch
we're at 5 min, which is still bad but this is a pathological case.

The cut off at 20 uses was chosen by looking at other cut-offs in LLVM
for user scanning. It's probably too high, but does the job and is very
unlikely to regress anything.

Fixes PR33900.

* I'm impatient and aborted after 15 minutes, on the bug report it was
  killed after 2h.

llvm-svn: 308891