nir/algebraic: Add lowering for ldexp
nir/algebraic: Add lowering for ldexp

The algorithm used is different from both the naive suggestion from the
GLSL spec and the one used in GLSL IR today.  Unfortunately, the GLSL IR
implementation that we have today doesn't handle denormals (for those that
care) or the case where the float source is +-inf.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler