[I slightly improve the patch covering a few more cases where
tree-tailcall gives up, especially with -O1 and -Os.
Here's the updated version.]
Give better error messages for musttail
When musttail is set, make tree-tailcall give error messages
when it cannot handle a call. This avoids vagu
When musttail is set, make tree-tailcall to give error messages
when it cannot handle a call. This avoids vague "other reasons"
error messages later at expand time.
This doesn't always work, for example when find_tail_call
walking gives up because the control flow is too complicated
then it won't
