gallivm: Increase lp_build_rsqrt() precision.
gallivm: Increase lp_build_rsqrt() precision.

Add an iteration step, which makes rqsqrt precision go from 12bits to
24, and fixes RSQ/NRM test case of PSPrecision/VSPrevision DCTs.

There are no uses of this function outside shader translation.

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