Changes

[mlir][Linalg] Add verification checks to disallow illegal reshape ops.

The existing verification of reshape ops in linalg (linalg.reshape and
linalg.tensor_reshape) allows specification of illegal ops, where
- A dynamic dimension is expanded into multiple dynamic
  dimensions. This is ill-specified.
- A static dimension is expanded into dynamic dimension or viceversa,
- The product of extents of the static dimensions in the expanded type
  doesnt match the static dimension of the collapsed type.
Making all of these illegal. This also implies that some pessimization
in canonicalization due to incomplete semantics of the operation can
be dropped.

Differential Revision: https://reviews.llvm.org/D93724

[Signal] Re-raise SIGPIPE if the handler is uninstalled

Instead of falling through to RunSignalHandlers after the SIGPIPE
handler is uninstalled and we get a SIGPIPE, re-raise the signal, just
like we do for other IntSigs.

This was discussed and informally OK'd here:

https://reviews.llvm.org/rG9a3f892d018238dce5181e458905311db8e682f5#856804