[esan|wset] Optionally assume intra-cache-line accesses

Summary:
Adds an option -esan-assume-intra-cache-line which causes esan to assume
that a single memory access touches just one cache line, even if it is not
aligned, for better performance at a potential accuracy cost.  Experiments
show that the performance difference can be 2x or more, and accuracy loss
is typically negligible, so we turn this on by default.  This currently
applies just to the working set tool.

Reviewers: aizatsky

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20978

llvm-svn: 271743