ir_constant: Don't assert on out-of-bounds array accesses
ir_constant: Don't assert on out-of-bounds array accesses

Several optimization paths, including constant folding, can lead to
accessing an ir_constant array with an out of bounds index.  The GLSL
spec lets us produce "undefined" results, but it does not let us
crash.

Fixes piglit test case glsl-array-bounds-01 and glsl-array-bounds-03.

bkaradzic/glsl-optimizer,metora/MesaGLSLCompiler,adobe/glsl2agal,zeux/glsl-optimizer,benaadams/glsl-optimizer,adobe/glsl2agal,mcanthony/glsl-optimizer,KTXSoftware/glsl2agal,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,adobe/glsl2agal,KTXSoftware/glsl2agal,mapbox/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,adobe/glsl2agal,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,adobe/glsl2agal,zz85/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer