[InstCombine] fold shift-right-by-constant with shift-right-of-constant operand

  (C2 >> X) >> C1 --> (C2 >> C1) >> X

The shift-left form of this transform has existed since:
16f18ed7

...but it applies to matching shift right opcodes too:
https://alive2.llvm.org/ce/z/c5eQms