[inductor] Optimize away zero-element loads (#107074)
Fixes #107066, closes #107008
This replaces loads to zero-element `Loops` or `Buffer`s with `ops.constant`
calls. This both avoids the issue of masked loads under triton, and also means
the buffer is not listed as a dependency for downstream users which may improve
performance generally.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107074
Approved by: https://github.com/davidberard98
