tint: Use "demote-to-helper" semantics for discard

Discard statements no longer affect the behavior or uniformity
analysis. Update the resolver, validator, and several tests to reflect
this.

Some E2E tests were removed as they had loops that are now considered
to be infinite.

Use the DemoteToHelper transform to emulate the correct semantics on
platforms where discard is (or may) terminate the invocation in a
manner that would affect derivative operations.

We no longer need the UnwindDiscardFunctions transform for HLSL, which
already implements the correct semantics. However, we still run the
DemoteToHelper transform for the HLSL backend due to issues with FXC's
handling of discard statements (see crbug.com/tint/1118).

Fixed: tint:1723
Change-Id: Ib49ff187919ae81c4af8675e1b66acd57e2ff7d2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/109003
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>

test/tint/unittest/reader/spirv/SpvParserCFGTest_EmitBody_Kill_InsideLoop.spvasm

@@ -1,55 +0,0 @@
-; Test: SpvParserCFGTest_EmitBody_Kill_InsideLoop.spvasm
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