R600: Use lower-case for EnableIRStructurizer feature

llc converts all values passed to -mattr= to lowercase, so this
enables us to toggle this feature when using llc.
R600: Add a SubtargetFeatture for disabling the ifcvt pass.

This is useful when writing test cases for the AMDIL structurizer.
R600: Fix a crash in the AMDILCFGStrucurizer

The ifPatternMatch() function was not correctly reporting the number
of matches in some cases.
R600: Enable the IR structurizer by default