cmd/compile: make tabs narrow in src column of ssa.html

Too deeply nested code is hard to fit in ssa.html.
This CL reduces the tab size to 4 characters.

Change-Id: I08643b0868bce3439567084c7d701654655f23d7
Reviewed-on: https://go-review.googlesource.com/c/142857Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/html.go

@@ -109,6 +109,12 @@ code, pre, .lines, .ast {
     font-size: 12px;
 }
 
+pre {
+    -moz-tab-size: 4;
+    -o-tab-size:   4;
+    tab-size:      4;
+}
+
 .allow-x-scroll {
     overflow-x: scroll;
 }