[CostModel] fix cost calc bug for sadd/ssub with overflow

As noted in D90554, there's an opcode typo in using an easily
misused cost model API: getCmpSelInstrCost(). Beyond that, the
assumed sequence of ops is questionable, but that would be
another patch.

My guess is that the x86 test diffs show that we are probably
wrong both before and after this change, so there will be no
practical difference.
As an example, I tried this test which shows a cost of '7'
either way:

  define <4 x i32> @sadd(<4 x i32> %va, <4 x i32> %vb) {
    %V4I32  = call {<4 x i32>, <4 x i1>}  @llvm.sadd.with.overflow.v4i32(<4 x i32> %va, <4 x i32> %vb)
    %ov = extractvalue {<4 x i32>, <4 x i1>} %V4I32, 1
    %r = extractvalue {<4 x i32>, <4 x i1>} %V4I32, 0
    %z = select <4 x i1> %ov, <4 x i32> <i32 42, i32 42, i32 42, i32 42>, <4 x i32> %r
    ret <4 x i32> %z
  }

  $ llc -o - sadd.ll -mattr=avx
        vpaddd  %xmm1, %xmm0, %xmm2
        vpcmpgtd        %xmm2, %xmm0, %xmm0
        vpxor   %xmm0, %xmm1, %xmm0
        vblendvps       %xmm0, LCPI0_0(%rip), %xmm2, %xmm0a

Differential Revision: https://reviews.llvm.org/D90681