LLVM_Util: add support for masked operations (1248)

Work by Alex Wells.

This will be used extensively by the SIMD batched shading.

* Added support for masked operations by managing a stack of masks
  that were created by conditionals or control flow. Detailed
  explanation of the Masking Strategy in llvm_util.h.

* Added support for logically masked functions (handling of masked
  return from inside OSL functions)

* Added support for masked loops (handling of masked break and or
  masked continue).

* Added support for masked exit (handling of masked exit from entire
  shader).

* Added support for management of when masking is required with a
  ScopedMasking object to allow nesting during code gen.

* op_store (and other store variations) respect the current masking
  requirement and current top of the mask stack (not just the top
  entry, but a combined view of the entire mask stack).

* Added support for results of conditional operations to remain
  boolean.  Most masking operations are expecting vector of bool
  values, not a vector of integers., e.g. when vector width is 16, for
  scalar or <16 x i1> for wide. When a conditional result is used by
  another operation a numerical value it is automatically converted to
  an integer.

* Added `llvm::Type *type_native_mask() const;` because different
  ISA's may have different representations of a mask from llvm's
  vector of bits that comparison operations emit.  And we need varying
  boolean symbols to have the correct data type (size) on the stack
  and in data structures.

* Added gather and scatter operations.

* Added helpers to convert from bool<->int, native_mask<->llvm_mask

Signed-off-by: Larry Gritz <[email protected]>