[DAG] Combine insert(shuffle(load), load, 0) into a single load

Given an insert of a scalar load into a vector shuffle with mask
u,0,1,2,3,4,5,6 or 1,2,3,4,5,6,7,u (depending on the insert index),
it can be more profitable to convert to a single load and avoid the
shuffles. This adds a DAG combine for it, providing the new load is
still fast.

Differential Revision: https://reviews.llvm.org/D151029