Remove -join-physregs from the test suite.

This option has been disabled for a while, and it is going away so I can
clean up the coalescer code.

The tests that required physreg joining to be enabled were almost all of
the form "tiny function with interference between arguments and return
value". Such functions are usually inlined in the real world.

The problem exposed by phys_subreg_coalesce-3.ll is real, but fairly
rare.

llvm-svn: 157027