Fix banded-diagonal `Lmul`/`Rmul` copy #666

Job	Run time
Documentation	1m 7s
pre_job	7s
Julia lts - ubuntu-latest - pull_request	4m 59s
Julia lts - macOS-latest - pull_request	3m 10s
Julia lts - windows-latest - pull_request	5m 44s
Julia 1 - ubuntu-latest - pull_request	7m 19s
Julia 1 - macOS-latest - pull_request	4m 37s
Julia 1 - windows-latest - pull_request	7m 46s
	34m 49s

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