glsl: Fix continue statements in do-while loops.

From the GLSL 4.40 spec, section 6.4 (Jumps):

    The continue jump is used only in loops. It skips the remainder of
    the body of the inner most loop of which it is inside. For while
    and do-while loops, this jump is to the next evaluation of the
    loop condition-expression from which the loop continues as
    previously defined.

Previously, we incorrectly treated a "continue" statement as jumping
to the top of a do-while loop.

This patch fixes the problem by replicating the loop condition when
converting the "continue" statement to IR.  (We already do a similar
thing in "for" loops, to ensure that "continue" causes the loop
expression to be executed).

Fixes piglit tests:
- glsl-fs-continue-inside-do-while.shader_test
- glsl-vs-continue-inside-do-while.shader_test

Cc: mesa-stable@lists.freedesktop.org