intrinsic_op: restrict math operations to actually implemented overloads

[rainers]

also uses simpler argument check for bsf, bsr, bswap

This should unbreak the master build.

[dlang-bot]

Thanks for your pull request, @rainers!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10665"

[ibuclaw]

Fetching libasync 0.8.3 (getting selected version)...
Failed to download package libasync from https://code.dlang.org/packages/libasync/0.8.3.zip
posix.mak:774: recipe for target 'dpl-docs' failed
make: *** [dpl-docs] Error 2

[rainers]

Failed to download package libasync from https://code.dlang.org/packages/libasync/0.8.3.zip

build retriggered.

[rainers]

readded automerge, as I just force-pushed to retrigger the doc tester.