cmd/compile: get rid of redundant Type helper functions
Replace Isfixedarray, Isslice, and Isinter with the IsArray, IsSlice,
and IsInterface methods added for SSA. Rewrite performed mechanically
using gofmt -w -r "Isfoo(t) -> t.IsFoo()".
Because the IsFoo methods panic when given a nil pointer, a handful of
call sites had to be modified to check for nil Type values. These
aren't strictly necessary, because nil Type values should only occur
in invalid Go source programs, so it would be okay if we panicked on
them and gave up type checking the rest of the package. However, there
are a couple regress tests that expect we continue, so add checks to
keep those tests passing. (See #15029.)