[DAGCombine] Fold (store (insert_elt (load p)) x p) -> (store x)

If we have a store of a load with no other uses in between it, it's
considered dead and is removed. So sometimes when legalizing a fixed
length vector store of an insert, we end up producing better code
through scalarization than without.
An example is the follow below:

  %a = load <4 x i64>, ptr %x
  %b = insertelement <4 x i64> %a, i64 %y, i32 2
  store <4 x i64> %b, ptr %x

If this is scalarized, then DAGCombine successfully removes 3 of the 4
stores which are considered dead, and on RISC-V we get:

  sd a1, 16(a0)

However if we make the vector type legal (-mattr=+v), then we lose the
optimisation because we don't scalarize it.

This patch attempts to recover the optimisation for vectors by
identifying patterns where we store a load with a single insert
inbetween, replacing it with a scalar store of the inserted element.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D152276

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