Fix banded-diagonal `Lmul`/`Rmul` copy #668

Job	Run time
Documentation	1m 8s
pre_job	9s
Julia lts - ubuntu-latest - pull_request	5m 6s
Julia lts - macOS-latest - pull_request	3m 59s
Julia lts - windows-latest - pull_request	5m 27s
Julia 1 - ubuntu-latest - pull_request	7m 23s
Julia 1 - macOS-latest - pull_request	6m 7s
Julia 1 - windows-latest - pull_request	8m 47s
	38m 6s

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