radeonsi: fix hang caused by for loop with exec=0 in LS and ES

radeonsi: fix hang caused by for loop with exec=0 in LS and ES

LLVM expects that exec != 0 when entering loops and generates this code
that becomes an infinite loop if exec == 0:

BB5_1:
    vcc_lo = (inverted terminating condition)
    s_and_b32 vcc_lo, exec_lo, vcc_lo
    s_cbranch_vccnz BB5_3    // jump if vcc != 0 (break statement)
    // ... loop body ...
    s_branch BB5_1
BB5_3:

For non-monolithic VS before TCS, VS before GS, and TES before GS,
we set exec = (thread enabledmask), which sets 0 for HS-only and GS-only
waves, causing the infinite loop condition above.

Fix it as follows:
- set exec = ~0 at the beginning
- wrap the whole shader (LS and ES) in a conditional block, so that HS-only
  and GS-only waves jump over it and never enter such a loop

The TES before GS hang can be reproduced by gfxbench:
    testfw_app --gfx egl -w 1920 -h 1080 --gl_api gles -t gl_tess

Fixes: 68d6d097f15 - radeonsi/gfx9: add GFX9 and VEGA10 enums

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8344>