[SLP] no need to generate extract for in-tree uses for original scalar instruction.

Before https://github.com/llvm/llvm-project/commit/77a609b55636dc540090ef9105c60a99cfdbd1dd,
we always skip in-tree uses of the vectorized scalars in `buildExternalUses()`,
that commit handles the case that if the in-tree use is scalar operand in vectorized instruction,
we need to generate extract for these in-tree uses.

in-tree uses remain as scalar in vectorized instructions can be 3 cases:
- The pointer operand of vectorized LoadInst uses an in-tree scalar
- The pointer operand of vectorized StoreInst uses an in-tree scalar
- The scalar argument of vector form intrinsic uses an in-tree scalar

Generating extract for in-tree uses for vectorized instructions are implemented in `BoUpSLP::vectorizeTree()`:
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11497-L11506
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11542-L11551
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11657-L11667

However, https://github.com/llvm/llvm-project/commit/77a609b55636dc540090ef9105c60a99cfdbd1dd
not only generates extract for vectorized instructions,
but also generates extract for original scalar instructions.
There is no need to generate extract for origin scalar instrutions,
as these scalar instructions will be replaced by vector instructions and get erased later.

This patch replaces extracts for original scalar instructions with corresponding vectorized instructions,
and remove
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11497-L11506
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11542-L11551
- https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L11657-L11667
extracts.