isNoWrap has exactly one caller which handles Assume = true separately,
but too conservatively. Instead, pass Assume to isNoWrap, so it is
threaded into getPtrStride, which has the correct handling for the
Assume flag. Also note that the Stride == 1 check in isNoWrap is
incorrect: getPtrStride returns Strides == 1 or -1, except when
isNoWrapAddRec or Assume are true, assuming ShouldCheckWrap is true; we
can include the case of -1 Stride, and when isNoWrapAddRec is true. With
this change, passing Assume = true to getPtrStride could return a
non-unit stride, and we correctly handle that case as well.
Currently generateOverflowCheck always creates code for Step being
negative and positive, followed by a select at the end depending on
Step's sign.
This patch updates the code to only create either the checks for step
being positive or negative, if the sign is known.
Follow-up to D116696.
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D116747
