mesa: fix prog_optimize.c assertions triggered by SWZ opcode
The SWZ instruction can have swizzle terms >4 (SWIZZLE_ZERO, SWIZZLE_ONE).
These swizzle terms caused a few assertions to fail.
This started happening after the commit "mesa: Actually use the Mesa IR
optimizer for ARB programs." when replaying some apitrace files.
A new piglit test (tests/asmparsertest/shaders/ARBfp1.0/swz-08.txt)
exercises this.
Cc: "10.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>