[InstCombine] visitTrunc - trunc (lshr (sext A), C) --> (ashr A, C) non-uniform support

This came from @lebedev.ri's suggestion to use m_SpecificInt_ICMP for D88429 - since I was going to change the m_APInt to m_Constant for that patch I thought I would do it for the only other user of the APInt first.

I've added a ConstantExpr::getUMin helper - its trivial to add UMAX/SMIN/SMAX but thought I'd wait until we have use cases.

Differential Revision: https://reviews.llvm.org/D88475