(jump_optimize): Delete insns that set registers that are not used elsewhere.

Fix some bugs in "if (...) x = a; else x = b;" code.
Remove uses of reorder_insns.
Add code to make scc insns for "if (...) x = exp;" if EXP is simple enough.
Optimize "if (...) x = b;" if jumps very expensive; don't use a REG as the
older value of X since it doesn't help and can hurt.
Correctly place generated insns in above case.

From-SVN: r2191