[InstCombine] FoldShiftByConstant - consistently use ConstantExpr in...

[InstCombine] FoldShiftByConstant - consistently use ConstantExpr in logicalshift(trunc(shift(x,c1)),c2) fold. NFCI.

This still only gets used for scalar types but now always uses ConstantExpr in preparation for vector support - it was using APInt methods in some places.