[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.

[X86][CGP] Reduce memcmp() expansion to 2 load pairs (PR33914)

D35067/rL308322 attempted to support up to 4 load pairs for memcmp
inlining which resulted in regressions for some optimized libc memcmp
implementations (PR33914).

Until we can match these more optimal cases, this patch reduces the
memcmp expansion to a maximum of 2 load pairs (which matches what we
do for -Os).

This patch should be considered for the 5.0.0 release branch as well

Differential Revision: https://reviews.llvm.org/D35830