[InstCombinePHI] Partial simplification of identity operations.
Consider this code:
%i = phi i32 [ 0, %if.then ], [ %c, %if.else ] %add = add nsw i32 %i, %b ...
In this common case the add can be moved to the %if.else basic block, because
adding zero is an identity operation. If we go though %if.then branch it's
always a win, because add is not executed; if not, the number of instructions
stays the same.
This pattern applies also to other instructions like sub, shl, shr, ashr | 0,
mul, sdiv, div | 1.
Patch by Jakub Kuderski!