tgsi: add tgsi_ureg, a simplified tgsi shader builder

tgsi: add tgsi_ureg, a simplified tgsi shader builder

This is modelled on the nice & easy-to-use facilities we had
for building shaders in mesa, eg. in texenvprogram.c and friends.

Key points include pass-by-value register structs that can be manipulated
in a functional style, eg:

   negate(swizzle(reg, X,X,X,X))

and per-opcode instruction functions, eg:

   emit_MOV( p, writemask(dst, 0x1), negate(src));

and similar.

Additionally, the interface allows mixed emit of instructions and decls,
which are sorted out internally to obey TGSI ordering.

Immediates may be emitted at any time and are scanned against existing
immediates to try and reduce redundancy.

Not all TGSI functionality is accessible through this interface, but
most or all of what mesa uses should be.