[SLP][X86]Fix the cost of reused gathers/buildvectors and floats insert.

[SLP][X86]Fix the cost of reused gathers/buildvectors and floats insert.

There are 2 problems in the cost estimation for buildvector/gather.
1. If the buildvector/gather node is the same as another one node, need
   to estimate the cost of this node as 0.
2. The cost of inserting float point register to non-poison vector is
   not 0, it should not be considered free.

Differential Revision: https://reviews.llvm.org/D148801