[SystemZ] Use MVC for memcpy

Use MVC for memcpy in cases where a single MVC is enough.  Using MVC is
a win for longer copies too, but I'll leave that for later.
Remove redundant #include.
[SystemZ] Remove unwanted part from last commit

I was originally going to use MVC for memmove too, but that's less of
a clear win.  Remove some accidental left-overs in the previous commit.
[LSan] Use overridable hook to report error summary
Revert "Proper va_arg/va_copy lowering on win64"

This reverts commit 2b52880592a525cfe04d8f9008a35da8c2ea94c3.

Needs review.
Reuse %rax after calling __chkstk on win64

Reapply this as I reverted the wrong commit.
InstCombine: Fold X-C1 <u 2 -> (X & -2) == C1

Back in r179493 we determined that two transforms collided with each
other.  The fix back then was to reorder the transforms so that the
preferred transform would give it a try and then we would try the
secondary transform.  However, it was noted that the best approach would
canonicalize one transform into the other, removing the collision and
allowing us to optimize IR given to us in that form.