[InstCombine] Optimize compares with multiple selects as operands

In case of a comparison with two select instructions having the same
condition, check whether one of the resulting branches can be simplified.
If so, just compare the other branch and select the appropriate result.
For example:

    %tmp1 = select i1 %cmp, i32 %y, i32 %x
    %tmp2 = select i1 %cmp, i32 %z, i32 %x
    %cmp2 = icmp slt i32 %tmp2, %tmp1

The icmp will result false for the false value of selects and the result
will depend upon the comparison of true values of selects if %cmp is
true. Thus, transform this into:

    %cmp = icmp slt i32 %y, %z
    %sel = select i1 %cond, i1 %cmp, i1 false

Differential Revision: https://reviews.llvm.org/D150360