glsl/loops: consolidate bounded loop handling into a lowering pass.

glsl/loops: consolidate bounded loop handling into a lowering pass.

Previously, all of the back-ends (ir_to_mesa, st_glsl_to_tgsi, and the
i965 fs and vec4 visitors) had nearly identical logic for handling
bounded loops.  This replaces the duplicate logic with an equivalent
lowering pass that is used by all the back-ends.

Note: on i965, there is a slight increase in instruction count.  For
example, a loop like this:

    for (int i = 0; i < 100; i++) {
      total += i;
    }

would previously compile down to this (vec4) native code:

          mov(8)       g4<1>.xD 0D
          mov(8)       g8<1>.xD 0D
    loop:
          cmp.ge.f0(8) null     g8<4;4,1>.xD 100D
    (+f0) break(8)
          add(8)       g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
          add(8)       g8<1>.xD g8<4;4,1>.xD 1D
          add(8)       g4<1>.xD g4<4;4,1>.xD 1D
          while(8) loop

After this patch, the "(+f0) break(8)" turns into:

    (+f0) if(8)
          break(8)
          endif(8)

because the back-end isn't smart enough to recognize that "if
(condition) break;" can be done using a conditional break instruction.
However, it should be relatively easy for a future peephole
optimization to properly optimize this.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>