GenTree* shiftConst = m_compiler->gtNewIconNode(31, TYP_INT);
GenTree* sar        = m_compiler->gtNewOperNode(GT_RSH, select->TypeGet(), relopOp1, shiftConst);
CheckImmedAndMakeContained(sar, shiftConst); // false

CheckImmedAndMakeContained calls into IsContainableImmed which on arm doesn't handle shifts and returns false so that shiftConst doesn't get contained even though it could.

The weird thing is that the higher level function ContainCheckNode handles it. That's because it doesn't actually call into CheckImmedAndMakeContained. Meanwhile for other nodes like add the higher level function does call into lower level CheckImmedAndMakeContained:

Add:
ContainCheckNode -> ContainCheckBinary -> CheckImmedAndMakeContained -> IsContainableImmed -> MakeSrcContained

Rsh:
ContainCheckNode -> ContainCheckShiftRotate -> shiftBy->IsCnsIntOrI -> MakeSrcContained

Maybe ContainCheckShiftRotate should call CheckImmedAndMakeContained which should handle shifts?