target-xtensa: don't generate dead code to access invalid SRs

This fixes the following test failure caused by access to undefined SR:

    qemu-system-xtensa -M sim -cpu dc232b -nographic -semihosting  -kernel ./test_sr.tst
    QEMU 1.4.50 monitor - type 'help' for more information
    (qemu) QEMU 1.4.50 monitor - type 'help' for more information
    (qemu) qemu-system-xtensa: tcg/tcg.c:1673: temp_save: Assertion `s->temps[temp].val_type == 2 || s->temps[temp].fixed_reg' failed.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>