btf: fix up inflated types immediately if possible

[lmb]

Converting []rawType into []Type currently creates a lot of temporary
data in the form of fixups. They are necessary since a type may reference
itself, for example in a linked list:

struct node {
    struct node* next;
    ...
}

No matter which order we encode struct node and struct node* in, we
can't resolve one of the types until we've inflated both.

However, such self referential types are rare. Most of the time we
are referencing an already inflated type. We can use this to our
advantage and perform fixups opportunistically. At the same time
we can split out type assertions from fixups, since most fixups
don't care about types at all.

name            old time/op    new time/op    delta
ParseVmlinux-4    68.9ms ± 2%    58.4ms ± 1%  -15.22%  (p=0.029 n=4+4)

name            old alloc/op   new alloc/op   delta
ParseVmlinux-4    48.4MB ± 0%    45.2MB ± 0%   -6.69%  (p=0.029 n=4+4)

name            old allocs/op  new allocs/op  delta
ParseVmlinux-4      721k ± 0%      721k ± 0%     ~     (p=0.057 n=4+4)

Updates #542

[lmb]

@paulcacheux looking at your commit I realised that there is even more improvements to be had. Please take a look.