[release-branch.go1.8] cmd/compile: add missing WBs for reflect.{Slice,String}Header...

[release-branch.go1.8] cmd/compile: add missing WBs for reflect.{Slice,String}Header.Data

Fixes #19168.

(*state).insertWBstore needed to be tweaked for backporting so that
store reflect.{Slice,String}Header.Data stores still fallthrough and
end the SSA block. This wasn't necessary at master because of CL
36834.

Change-Id: I3f4fcc0b189c53819ac29ef8de86fdad76a17488
Reviewed-on: https://go-review.googlesource.com/39615
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Austin Clements <austin@google.com>