[X86] Merge combineX86ShuffleChainWithExtract into combineX86ShufflesRecursively

[RKSimon]

combineX86ShufflesRecursively can now handle changes in the size of the root node with each recursion, this should allow us to handle length reducing shuffles (EXTRACT_SUBVECTOR / TRUNCATE / COMPRESS etc.) as we merge shuffles up a chain.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

combineX86ShufflesRecursively can now handle changes in the size of the root node with each recursion, this should allow us to handle length reducing shuffles (EXTRACT_SUBVECTOR / TRUNCATE / COMPRESS etc.) as we merge shuffles up a chain.