[EarlyCSE] Equivalent SELECTs should hash equally

DenseMap<SimpleValue> assumes that, if its isEqual method returns true
for two elements, then its getHashValue method must return the same value
for them. This invariant is broken when one SELECT node is a min/max
operation, and the other can be transformed into an equivalent min/max by
inverting its predicate and swapping its operands. This patch fixes an
assertion failure that would occur intermittently while compiling the
following IR:

    define i32 @t(i32 %i) {
      %cmp = icmp sle i32 0, %i
      %twin1 = select i1 %cmp, i32 %i, i32 0
      %cmpinv = icmp sgt i32 0, %i
      %twin2 = select i1 %cmpinv,  i32 0, i32 %i
      %sink = add i32 %twin1, %twin2
      ret i32 %sink
    }

Differential Revision: https://reviews.llvm.org/D86843