freedreno/ir3: fix valgrind complaint with STLW

freedreno/ir3: fix valgrind complaint with STLW

The instruction has 3 src regs, so `instr->regs[0..3]` are valid, but
`instr->regs[4]` is not.

```
Test case 'dEQP-GLES31.functional.shaders.linkage.es31.tessellation.varying.rules.output_superfluous_declaration'..
==29239== Invalid read of size 8
==29239==    at 0x5BE9CDC: emit_cat6 (ir3.c:841)
==29239==    by 0x5BEA1BF: ir3_assemble (ir3.c:921)
==29239==    by 0x5BDF0A7: ir3_shader_assemble (ir3_shader.c:133)
==29239==    by 0x5BDF193: assemble_variant (ir3_shader.c:162)
==29239==    by 0x5BDF407: create_variant (ir3_shader.c:215)
==29239==    by 0x5BDF4DB: shader_variant (ir3_shader.c:241)
==29239==    by 0x5BDF553: ir3_shader_get_variant (ir3_shader.c:257)
==29239==    by 0x5BA85F7: ir3_shader_variant (ir3_gallium.c:80)
==29239==    by 0x5BA7703: ir3_cache_lookup (ir3_cache.c:96)
==29239==    by 0x5B8B8B3: fd6_emit_get_prog (fd6_emit.h:119)
==29239==    by 0x5B8C137: fd6_draw_vbo (fd6_draw.c:186)
==29239==    by 0x5BB1FBB: fd_draw_vbo (freedreno_draw.c:290)
==29239==  Address 0xb97f2d0 is 0 bytes after a block of size 240 alloc'd
==29239==    at 0x4848D54: malloc (in /usr/lib/aarch64-linux-gnu/valgrind/vgpreload_memcheck-arm64-linux.so)
==29239==    by 0x61BD35B: ralloc_size (ralloc.c:119)
==29239==    by 0x61BD41B: rzalloc_size (ralloc.c:151)
==29239==    by 0x5BE599B: ir3_alloc (ir3.c:45)
==29239==    by 0x5BEA583: instr_create (ir3.c:984)
==29239==    by 0x5BEA5DF: ir3_instr_create2 (ir3.c:1000)
==29239==    by 0x5BEE317: ir3_STLW (ir3.h:1431)
==29239==    by 0x5BF12D3: emit_intrinsic_store_shared_ir3 (ir3_compiler_nir.c:903)
==29239==    by 0x5BF418B: emit_intrinsic (ir3_compiler_nir.c:1802)
==29239==    by 0x5BF5D07: emit_instr (ir3_compiler_nir.c:2339)
==29239==    by 0x5BF603F: emit_block (ir3_compiler_nir.c:2426)
==29239==    by 0x5BF624B: emit_cf_list (ir3_compiler_nir.c:2474)
==29239==
```

Probably this only triggers in non-optimized builds?

Fixes: 1f3b52ce503 ("freedreno/a6xx: Add register offset for STG/LDG")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Anholt <eric@anholt.net>