This PR removes the old `nocapture` attribute, replacing it with the new
`captures` attribute introduced in #116990. This change is
intended to be essentially NFC, replacing existing uses of `nocapture`
with `captures(none)` without adding any new analysis capabilities.
Making use of non-`none` values is left for a followup.
Some notes:
* `nocapture` will be upgraded to `captures(none)` by the bitcode
reader.
* `nocapture` will also be upgraded by the textual IR reader. This is to
make it easier to use old IR files and somewhat reduce the test churn in
this PR.
* Helper APIs like `doesNotCapture()` will check for `captures(none)`.
* MLIR import will convert `captures(none)` into an `llvm.nocapture`
attribute. The representation in the LLVM IR dialect should be updated
separately.
Builds on #67982 which recently introduced the nneg flag on a zext
instruction. InstCombine is one of our largest canonicalizers of zext
from non-negative sext instructions, so set the flag there.
This was supposed to document the new PM limitation but
was deleted in fb4113ef0c8b2c5e5e2817e9ca14fb57a6d252be
Switch to generated checks since that's more reliable than XFAIL, and
just preserve the preferred results as comments.
