[InstCombine] Folding of shifts by the sum of positive values

This patch introduces the combine:

(C1 shift (A add C2)) -> ((C1 shift C2) shift A)
iff A and C2 are both positive

If both A and C2 are know to be positive then we can safely split into 2 shifts, permitting the folding of the Inner shift.

Fix for the spec benchmark case mentioned by @nadav on PR15141 (assuming we can prove that the inputs as positive).

Differential Revision: https://reviews.llvm.org/D26000

llvm-svn: 285696