JIT: Remove TYP_BLK and TYP_LCLBLK (#83036)

JIT: Remove TYP_BLK and TYP_LCLBLK (#83036)

This PR allows TYP_STRUCT locals to have block layouts and replaces uses
of TYP_BLK and TYP_LCLBLK with such locals instead.

There is still an invariant that any struct parameter local (even SIMD) has a non-block layout.

Also fixes a bug related to GS cookie handle with jit32 GC encoder -- it is
not allowed to be at fp+0, so we need to insert some padding in some cases because
now that some GS cookie-requiring locals are TYP_STRUCT they can sometimes be
removed from the frame.

To fix a TP regression, also precache the 0-sized block layout in the layout table. This
layout is used by all non-x86 compilations because the outgoing arg area starts out
as a 0-sized block local that is always allocated.