HLSL: force FXC to never unroll loops

Emit the "[loop]" attribute on "for" and "while" so that FXC does not
attempt to unroll them. This is to work around an FXC bug where it fails
to unroll loops with gradient operations.

FXC ostensibly unrolls such loops because gradient operations require
uniform control flow, and loops that have varying iterations may
possibly not be uniform. Tint will eventually validate that control flow
is indeed uniform, so forcing FXC to avoid unrolling in these cases
should be fine.

Bug: tint:1112
Change-Id: I10077f8b62fbbb230a0003f3864c75a8fe0e1d18
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69880
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

test/statements/for/basic.wgsl.expected.hlsl

@@ -8,7 +8,7 @@ void some_loop_body() {
 
 void f() {
   {
-    for(int i = 0; (i < 5); i = (i + 1)) {
+    [loop] for(int i = 0; (i < 5); i = (i + 1)) {
       some_loop_body();
     }
   }

test/statements/for/complex.wgsl.expected.hlsl

@@ -10,7 +10,7 @@ void f() {
   int j = 0;
   {
     int i = 0;
-    while (true) {
+    [loop] while (true) {
       bool tint_tmp = (i < 5);
       if (tint_tmp) {
         tint_tmp = (j < 10);

test/statements/for/condition.wgsl.expected.hlsl

@@ -6,7 +6,7 @@ void unused_entry_point() {
 void f() {
   int i = 0;
   {
-    for(; (i < 4); ) {
+    [loop] for(; (i < 4); ) {
     }
   }
 }

test/statements/for/continuing.wgsl.expected.hlsl

@@ -6,7 +6,7 @@ void unused_entry_point() {
 void f() {
   int i = 0;
   {
-    for(; ; i = (i + 1)) {
+    [loop] for(; ; i = (i + 1)) {
     }
   }
 }

test/statements/for/empty.wgsl.expected.hlsl

@@ -5,7 +5,7 @@ void unused_entry_point() {
 
 void f() {
   {
-    for(; ; ) {
+    [loop] for(; ; ) {
     }
   }
 }

test/statements/for/initializer.wgsl.expected.hlsl

@@ -5,7 +5,7 @@ void unused_entry_point() {
 
 void f() {
   {
-    for(int i = 0; ; ) {
+    [loop] for(int i = 0; ; ) {
     }
   }
 }

test/statements/for/scoping.wgsl.expected.hlsl

@@ -5,7 +5,7 @@ void unused_entry_point() {
 
 void f() {
   {
-    for(int must_not_collide = 0; ; ) {
+    [loop] for(int must_not_collide = 0; ; ) {
     }
   }
   int must_not_collide = 0;