Fix handling of case_reg in switch statement.
Fix handling of case_reg in switch statement.

The back end functions gen_op(comparison) and gtst() might allocate
registers so case_reg should be left on the value stack while they
are called and set again afterwards.

This bug fix was first applied as ff3f9aa (20 Feb 2015), but the fix
was reverted by fc0fc6a (21 Sep 2016, "switch: collect case ranges
first, then generate code"). Here the fix is updated for the new code.

avih/tinycc,mirror/tinycc,mingodad/tinycc,metaeducation/tcc,code4game/tinycc,mingodad/tinycc,andreiw/tinycc,andreiw/tinycc,metaeducation/tcc,mingodad/tinycc,run4flat/tinycc,metaeducation/tcc,mirror/tinycc,mingodad/tinycc,avih/tinycc,code4game/tinycc,avih/tinycc,run4flat/tinycc,run4flat/tinycc,mirror/tinycc,code4game/tinycc,avih/tinycc,run4flat/tinycc,metaeducation/tcc,run4flat/tinycc,andreiw/tinycc,mirror/tinycc,metaeducation/tcc