[InstCombine][SSE4a] Fix assertion failure caused by unsafe dyn_casts on the...

[InstCombine][SSE4a] Fix assertion failure caused by unsafe dyn_casts on the operands of extrq/extrqi intrinsic calls.

This patch fixes an assertion failure caused by unsafe dynamic casts on the
constant operands of sse4a intrinsic calls to extrq/extrqi

The combine logic that simplifies sse4a extrq/extrqi intrinsic calls currently
checks if the input operands are constants. Internally, that logic relies on
dyn_casts of values returned by calls to method Constant::getAggregateElement.
However, method getAggregateElemet may return nullptr if the constant element
cannot be retrieved. So, all the dyn_casts can potentially fail. This is what
happens for example if a constexpr value is passed in input to an extrq/extrqi
intrinsic call.

This patch fixes the problem by using a dyn_cast_or_null (instead of a simple
dyn_cast) on the result of each call to Constant::getAggregateElement.

Added reproducible test cases to x86-sse4a.ll.

Differential Revision: https://reviews.llvm.org/D24256

llvm-svn: 280804