[InstCombine] Fix matchRotate bug when one operand is a ConstantExpr shift

This bug seems to be harmless in release builds, but will cause an error in UBSAN
builds or an assertion failure in debug builds.

When it gets to this opcode comparison, it assumes both of the operands are BinaryOperators,
but the prior m_LogicalShift will also match a ConstantExpr. The cast<BinaryOperator> will
assert in a debug build, or reading an invalid value for BinaryOp from memory with
((BinaryOperator*)constantExpr)->getOpcode() will cause an error in a UBSAN build.

The test I added will fail without this change in debug/UBSAN builds, but not in release.

Patch by: @AndrewScheidecker (Andrew Scheidecker)

Differential Revision: https://reviews.llvm.org/D58049

llvm-svn: 353736