[spirv-reader] Log error for infinite loop.

When attempting to propagate through a loop, if we find no exits then
the loop is infinite. Emit a nicer error message and terminate.

Fixed: 42251941
Change-Id: If911378497444f8bb61530393dd9d514de3f68e4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/267854
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Auto-Submit: dan sinclair <dsinclair@chromium.org>

src/tint/lang/spirv/reader/parser/parser.cc

diff --git a/src/tint/lang/spirv/reader/parser/parser.cc b/src/tint/lang/spirv/reader/parser/parser.cc
index 91e6cfe..1fbfd8e 100644
--- a/src/tint/lang/spirv/reader/parser/parser.cc
+++ b/src/tint/lang/spirv/reader/parser/parser.cc
@@ -1218,6 +1218,12 @@
             // control as we're jumping over the control to its parent control. (This is an
             // `if` inside a `loop` where the `if` is doing a `break`).
             if (ctrl->Exits().IsEmpty()) {
+                if (ctrl->Is<core::ir::Loop>()) {
+                    TINT_UNREACHABLE()
+                        << "loop detected with no exits which means it's an infinite loop. "
+                           "Infinite loops are not supported in WGSL";
+                }
+
                 TINT_ASSERT(ctrl->Is<core::ir::If>());
                 blk = ctrl->Block();
                 continue;