JIT: do early block merging in more cases (#86157)

JIT: do early block merging in more cases (#86157)

OSR and PGO both rely on the fact that the early flowgraph the JIT builds is
the same flowgraph as the one seen in a previous JIT complation of that method,
since there is metadata (patchpoint offset, pgo schema) that refers to the
flowgraph. Previous work here (#85860) didn't ensure this for enough cases.

In particular if Tier0 does not do early block merging, but OSR does, it's possible
for the OSR entry point to fall within a merged block range, which the JIT is not
prepared to handle. This lead to the asserts seen in #86125.

The fix is to enable early block merging unless we're truly in a minopts or
debug codegen mode (previous to this Tier0 would not merge unless it also
was instrumenting; now it will always merge).

An alternative fix would be to find the block containing the OSR entry IL
offset, scan its statements, and split the block at the right spot, but that
seemed more involved.

Fixes #86125.