[SLP] Fix for PR31880: shuffle and vectorize repeated scalar ops on extracted elements

Summary:
Currently most of the time vectors of extractelement instructions are
treated as scalars that must be gathered into vectors. But in some
cases, like when we have extractelement instructions from single vector
with different constant indeces or from 2 vectors of the same size, we
can treat this operations as shuffle of a single vector or blending of 2
vectors.
```
define <2 x i8> @g(<2 x i8> %x, <2 x i8> %y) {
  %x0 = extractelement <2 x i8> %x, i32 0
  %y1 = extractelement <2 x i8> %y, i32 1
  %x0x0 = mul i8 %x0, %x0
  %y1y1 = mul i8 %y1, %y1
  %ins1 = insertelement <2 x i8> undef, i8 %x0x0, i32 0
  %ins2 = insertelement <2 x i8> %ins1, i8 %y1y1, i32 1
  ret <2 x i8> %ins2
}
```
can be converted to something like
```
define <2 x i8> @g(<2 x i8> %x, <2 x i8> %y) {
  %1 = shufflevector <2 x i8> %x, <2 x i8> %y, <2 x i32> <i32 0, i32 3>
  %2 = mul <2 x i8> %1, %1
  ret <2 x i8> %2
}
```
Currently this type of conversion is considered as high cost
transformation.

Reviewers: mzolotukhin, delena, mkuper, hfinkel, RKSimon

Subscribers: ashahid, RKSimon, spatel, llvm-commits

Differential Revision: https://reviews.llvm.org/D30200

llvm-svn: 309812