nir/spirv: Allow loop breaks in a switch body.

nir/spirv: Allow loop breaks in a switch body.

Per the SPIR-V spec 2.11 Structured Control Flow:

"The only blocks in a construct that can branch outside the construct are

...
- a break block for the innermost loop it is inside of.
..."

With

"Break block: A block containing a branch to the Merge Block of a loop header's merge instruction."

Note that it puts no restriction on not being in an if or switch within the innermost loop.

This passes the loop_break block to the switch body so it can properly detect loop breaks.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>