[PatternMatch][InstCombine] match a vector with constant expression element(s) as...

[PatternMatch][InstCombine] match a vector with constant expression element(s) as a constant expression

The InstCombine test is reduced from issue #56601. Without the more
liberal match for ConstantExpr, we try to rearrange constants in
Negator forever.

Alternatively, we could adjust the definition of m_ImmConstant to be
more conservative, but that's probably a larger patch, and I don't
see any downside to changing m_ConstantExpr. We never capture and
modify a ConstantExpr; transforms just want to avoid it.

Differential Revision: https://reviews.llvm.org/D130286