[Bug target/65249] unable to find a register to spill in class 'R0_REGS' when compiling protobuf on sh4

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65249

--- Comment #4 from Oleg Endo <olegendo at gcc dot gnu.org> ---
For some reason r0 is live throughout the whole basic block where the problem
occures.  The basic block starts with:

(note 36 35 38 5 [bb 5] NOTE_INSN_BASIC_BLOCK)
(note 38 36 39 5 NOTE_INSN_DELETED)
(note 39 38 70 5 NOTE_INSN_DELETED)
(insn 70 39 133 5 (set (reg:SI 0 r0)
        (const_int 0 [0])) ccdkVmsq_1.out:48 252 {movsi_ie}
     (nil))

When using LRA, r0 is spilled to stack, and PIC_REG (r12) is copied to r0 for
the mem load.  r0 is then reloaded from stack, but rematerialized and converted
into a load of constant.

        mov     r12,r2
        add     r11,r2
        mov     #0,r0         <<<< 
        mov.l   .L25,r1
        mov.l   @r2,r2
        mov.l   r0,@r15       <<<<
        mov     r12,r0        <<<<
        mov.l   @(60,r10),r3
        mov.l   @r2,r7
        cmp/eq  r3,r7
        mov     #0,r3
        mov     #0,r7
        mov.l   @(r0,r1),r1   <<<<
        bf/s    .L16
        mov     #0,r0         <<<<

        add     #12,r15
        ...

If I'm not mistaken, the above can be done better as:
        mov     r11,r0
        mov.l   @(r0,r12),r2
        mov.l   .L25,r0
        mov.l   @(60,r10),r3
        mov.l   @r2,r7
        cmp/eq  r3,r7
        mov     #0,r3
        mov     #0,r7
        mov.l   @(r0,r12),r1
        bf/s    .L16
        mov     #0,r0

        add     #12,r15
        ...