[tsan] compile-time instrumentation: do not instrument a read if

[tsan] compile-time instrumentation: do not instrument a read if
a write to the same temp follows in the same BB.
Also add stats printing.

On Spec CPU2006 this optimization saves roughly 4% of instrumented reads
(which is 3% of all instrumented accesses):
Writes            : 161216
Reads             : 446458
Reads-before-write: 18295

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154418 91177308-0d34-0410-b5e6-96231b3b80d8