Treat the WorkSet used to find ephemeral values as double-ended

We need to make sure that we visit all operands of an instruction before moving
deeper in the operand graph. We had been pushing operands onto the back of the work
set, and popping them off the back as well, meaning that we might visit an
instruction before visiting all of its uses that sit in between it and the call
to @llvm.assume.

To provide an explicit example, given the following:
  %q0 = extractelement <4 x float> %rd, i32 0
  %q1 = extractelement <4 x float> %rd, i32 1
  %q2 = extractelement <4 x float> %rd, i32 2
  %q3 = extractelement <4 x float> %rd, i32 3
  %q4 = fadd float %q0, %q1
  %q5 = fadd float %q2, %q3
  %q6 = fadd float %q4, %q5
  %qi = fcmp olt float %q6, %q5
  call void @llvm.assume(i1 %qi)

%q5 is used by both %qi and %q6. When we visit %qi, it will be marked as
ephemeral, and we'll queue %q6 and %q5. %q6 will be marked as ephemeral and
we'll queue %q4 and %q5. Under the old system, we'd then visit %q4, which
would become ephemeral, %q1 and then %q0, which would become ephemeral as
well, and now we have a problem. We'd visit %rd, but it would not be marked as
ephemeral because we've not yet visited %q2 and %q3 (because we've not yet
visited %q5).

This will be covered by a test case in a follow-up commit that enables
ephemeral-value awareness in the SLP vectorizer.