optimization/2692: excessive compile time with optimization

[Richard Henderson]

FWIW, 3.1 20020326 "only" took 12 minutes to compile this
test case, and "only" used 45MB.  CPU users were

 expand                : 438.54 (63%) usr   0.11 ( 4%) sys 438.62 (62%) wall
 reload CSE regs       :  93.51 (14%) usr   0.01 ( 0%) sys  93.50 (13%) wall
 global alloc          :  77.31 (11%) usr   0.05 ( 2%) sys  77.38 (11%) wall
 regmove               :  46.59 ( 7%) usr   0.01 ( 0%) sys  46.56 ( 7%) wall


r~