added basic dataflow analysis to skslc

src/sksl/README

 Overview
 ========
 
 SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's 
 internal shading language. SkSL is, at its heart, a single standardized version
 of GLSL which avoids all of the various version and dialect differences found
 in GLSL "in the wild", but it does bring a few of its own changes to the table.
 
 Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V
 before handing it over to the graphics driver.
 
 Differences from GLSL
 =====================
 
 SkSL is based on GLSL 4.5. For the most part, write SkSL exactly as you would
 desktop GLSL, and the SkSL compiler will take care of version and dialect
 differences (for instance, you always use "in" and "out", and skslc will handle
 translating them to "varying" and "attribute" as appropriate). Be aware of the 
 following differences between SkSL and GLSL:
 
-* no #version or "precision" statement is required, and they will be ignored if
-  present
+* no #version statement is required, and will be ignored if present
 * the output color is sk_FragColor (do not declare it)
 * lowp, mediump, and highp are always permitted (but will only be respected if 
   you run on a GLES device)
 * you do not need to include ".0" to make a number a float (meaning that
   "vec2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would often
   have to be expressed "vec2(x, y) * 4.0". There is no performance penalty for 
   this, as the number is converted to a float at compile time)
 * type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported
 * some built-in functions and one or two rarely-used language features are not
   yet supported (sorry!)
 
 SkSL is still under development, and is expected to diverge further from GLSL
 over time.