[DAG] visitINSERT_VECTOR_ELT - refactor BUILD_VECTOR creation from INSERT_VECTOR_ELT...

[DAG] visitINSERT_VECTOR_ELT - refactor BUILD_VECTOR creation from INSERT_VECTOR_ELT chain.

D127595 added the ability to recurse up a (one-use) INSERT_VECTOR_ELT chain to create a BUILD_VECTOR before other combines manage to break the chain, something that is particularly bad in D127115.

The patch generalises this so it doesn't have to build the chain starting from the last element insertion, instead it can now start from any insertion and will recurse up the chain until it finds all elements or finds a UNDEF/BUILD_VECTOR/SCALAR_TO_VECTOR which represents that start of the chain.

Fixes several regressions in D127115