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/142857


Reviewed-by: David Chase <drchase@google.com>