[SLP] Fix lookahead operand reordering for splat loads.

Splat loads are inexpensive in X86. For a 2-lane vector we need just one
instruction: `movddup (%reg), xmm0`. Using the standard Splat score leads
to worse code. This patch adds a new score dedicated for splat loads.

Please note that a splat is usually three IR instructions:
- It is usually a load and 2 inserts:
 %ld = load double, double* %gep
 %ins1 = insertelement <2 x double> poison, double %ld, i32 0
 %ins2 = insertelement <2 x double> %ins1, double %ld, i32 1

- But it can also be a load, an insert and a shuffle:
 %ld = load double, double* %gep
 %ins = insertelement <2 x double> poison, double %ld, i32 0
 %shf = shufflevector <2 x double> %ins, <2 x double> poison, <2 x i32> zeroinitializer

Because of this some of the lit tests contain more IR instructions.

Differential Revision: https://reviews.llvm.org/D121354