As the name suggest, this CL adds many operations related
to arithmetic subgroup operations, i.e.:

  - Reduction / Inclusive Scan / Exclusive Scans for:

    - IntAdd, UIntAdd, FloatAdd
    - IntMul, UIntMul, FloatMul,
    - IntMin, UIntMin, FloatMin,
    - IntMax, UIntMax, FloatMax,
    - BitwiseAnd, BitwiseOr, BitwiseXor
    - LogicalAnd, LogicalOr, LogicalXor

The implementation uses a single template function to
implement all these, based on the fact that these are
all binary commutative operations.

NOTE: Only 32-bit values are supported.

To make scans efficient, a new Reactor operation, named
Blend() is introduced. It is used to mix two input vectors
using 4 3-bit indices (encoded in a single 16-bit value)
to select the result's lane values.

A new unit-test is added to ReactorUnittests to check its
behaviour. Unfortunately, the test takes about 2 minutes
on a fast workstation when doing a full scan, so it will
by default only check 1/11th of all possible values
(see comments in the patch for more details).

Also, Float4::positive_inf() and Float4::negative_inf()
methods were added, since trying to build Float4(INFINITY)
will trigger a DCHECK() in the Float4(float) constructor,
and the infinity values are required by the subgroup
floating-point scan operations.

Bug: b/142002682
Test: dEQP-VK.subgroups.arithmetic.*
Change-Id: I86f509fc47f7475ca126615ed698ee493ae835ef
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38929Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: David Turner <digit@google.com>