[X86] LowerShiftByScalarVariable - find splat patterns with...

[X86] LowerShiftByScalarVariable - find splat patterns with getSplatSourceVector instead of getSplatValue

This completes the removal of uses of SelectionDAG::getSplatValue started in D119090 - by avoiding extracting the splatted element we make it a lot easier to zero-extend the bottom 64-bits of the shift amount and fixes issues we had on 32-bit targets where i64 isn't legal.

I've removed the old version of getTargetVShiftNode that took the scalar shift amount argument and LowerRotate can finally efficiently handle vXi16 rotates-by-scalar (using the same code as general funnel-shifts).

The only regression we see is in the X86-AVX2 PR52719 test case in vector-shift-ashr-256.ll - this is now hitting the same problem as the X86-AVX1 case (failure to simplify a multi-use X86ISD::VBROADCAST_LOAD) which I intend to address in a follow up patch.