[InstCombine] Fix a cycle when folding fneg(select) with scalable vector types (#112465)

The two folding operations are causing a cycle for the following case
with
scalable vector types:

define <vscale x 2 x double> @test_fneg_select_abs(<vscale x 2 x i1>
%cond, <vscale x 2 x double> %b) {
%1 = select <vscale x 2 x i1> %cond, <vscale x 2 x double>
zeroinitializer, <vscale x 2 x double> %b
  %2 = fneg fast <vscale x 2 x double> %1
  ret <vscale x 2 x double> %2
}

1) fold fneg:  -(Cond ? C : Y) -> Cond ? -C : -Y

2) fold select: (Cond ? -X : -Y) -> -(Cond ? X : Y)

1) results in the following since '<vscale x 2 x double>
zeroinitializer' passes
the check for the immediate constant:

%.neg = fneg fast <vscale x 2 x double> zeroinitializer
%b.neg = fneg fast <vscale x 2 x double> %b
%1 = select fast <vscale x 2 x i1> %cond, <vscale x 2 x double> %.neg,
<vscale x 2 x double> %b.neg

and so we end up going back and forth between 1) and 2).

Attempt to fold scalable vector constants, so that we end up with a
splat instead:

define <vscale x 2 x double> @test_fneg_select_abs(<vscale x 2 x i1>
%cond, <vscale x 2 x double> %b) {
  %b.neg = fneg fast <vscale x 2 x double> %b
%1 = select fast <vscale x 2 x i1> %cond, <vscale x 2 x double>
shufflevector (<vscale x 2 x double> insertelement (<vscale x 2 x
double> poison, double -0.000000e+00, i64 0), <vscale x 2 x double>
poison, <vscale x 2 x i32> zeroinitializer), <vscale x 2 x double>
%b.neg
  ret <vscale x 2 x double> %1
}