[dev.ssa] cmd/compile: fix build

[dev.ssa] cmd/compile: fix build

CL 14337 made SSA support fixedbugs/issue9604b.go.
That test contains > 40k blocks.
This made the O(n^2) dom algorithm fail to terminate
in a reasonable length of time, breaking the build.

For the moment, cap the number of blocks
to fix the build.

This will be reverted when a more efficient
dom algorithm is put in place,
which will be soon.

Change-Id: Ia66c2629481d29d06655ec54d1deff076b0422c6
Reviewed-on: https://go-review.googlesource.com/14342
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>