[llc] (almost) remove `--print-machineinstrs`

Its effect could be achieved by
`-stop-after`,`-print-after`,`-print-after-all`. But a few tests need to
print MIR after ISel which could not be done with
`-print-after`/`-stop-after` since isel pass does not have commandline name.
That's the reason `--print-machineinstrs` is downgraded to
`--print-after-isel` in this patch. `--print-after-isel` could be
removed after we switch to new pass manager since isel pass would have a
commandline text name to use `print-after` or equivalent switches.

The motivation of this patch is to reduce tests dependency on
would-be-deprecated feature.

Reviewed By: arsenm, dsanders

Differential Revision: https://reviews.llvm.org/D83275