x/tools/internal/refactor/inline,x/tools/gopls: undo variadic elimination when it is safe to do so

[findleyr]

Follow up on this comment on a semi-automated CL: https://go-review.googlesource.com/c/tools/+/537475/1/gopls/internal/lsp/source/rename.go#973

The inliner should undo variadic elimination (packing variadic arguments as []T{...}) when it is safe to do so. As a result of the current logic, the inliner may introduce this redundancy at the callsite.

CC @adonovan

[adonovan]

FWIW, "Safe to do so" means there is exactly one use of the parameter in the callee body and it is the final argument of an ellipsis call f(param...).