[safestack] Rewrite isAllocaSafe using SCEV.

Use ScalarEvolution to calculate memory access bounds.
Handle function calls based on readnone/nocapture attributes.
Handle memory intrinsics with constant size.

This change improves both recall and precision of IsAllocaSafe.
See the new tests (ex. BitCastWide) for the kind of code that was wrongly
classified as safe.

SCEV efficiency seems to be limited by the fact the SafeStack runs late
(in CodeGenPrepare), and many loops are unrolled or otherwise not in LCSSA.

llvm-svn: 253083