[InstCombine] Shift amount reassociation: fixup constantexpr handling (PR42484)

I was actually wondering if there was some nicer way than m_Value()+cast,
but apparently what i was really "subconsciously" thinking about
was correctness issue.

hasNoUnsignedWrap()/hasNoUnsignedWrap() exist for Instruction,
not for BinaryOperator, so let's just use m_Instruction(),
thus both avoiding a cast, and a crash.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42484,
      https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15587

llvm-svn: 364915