[PatternMatch] Do not match constant expressions for binops

Currently, m_Mul() style matchers also match constant expressions.
This is a regular source of assertion failures (usually by trying
to do a match and then cast to Instruction or BinaryOperator) and
infinite combine loops. At the same time, I don't think this provides
useful optimization capabilities (all of the tests affected here are
regression tests for crashes / infinite loops).

Long term, all of these constant expressions (apart from possibly
add/sub) are slated for removal per
https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179
-- but doing those removals can itself expose new crashes and
infinite loops due to the current PatternMatch behavior.

Differential Revision: https://reviews.llvm.org/D156401