[DAGCombine] Handle promotion of shift with both operands the same

When promoting a shift, make sure we only fetch the second operand
after promoting the first. Load promotion may replace users of the
old load, and we don't want to be left with a dangling reference to
the old load instruction.

The crashing test case is from https://reviews.llvm.org/D126689#3553212.

Differential Revision: https://reviews.llvm.org/D126886