RISC-V: Don't prefer FP_REGS for integers

On RISC-V we can't store integers in floating-point registers as this is
forbidden by the ISA.  We've always disallowed this, but we were
setting the preferred mode to FP_REGS for some integer modes.  This
caused the LRA to blow up with some hard to read error messages.

This patch removes the prefered mode hook, as the right thing to do here
is nothing.

Thanks to Kito for finding the bug, and mpf for the fix.  See also
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79912>.

PR target/79912

From-SVN: r246283

gcc/ChangeLog

 2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>
 
+	PR target/79912
+	* config/riscv/riscv.c (riscv_preferred_reload_class): Remove.
+	(TARGET_PREFERRED_RELOAD_CLASS): Likewise.
+
+2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>
+
 	* config/riscv/riscv.c (riscv_print_operand): Use "fence
 	iorw,ow".
 	* config/riscv/sync.mc (mem_thread_fence_1): Use "fence

gcc/config/riscv/riscv.c

@@ -3629,16 +3629,6 @@ riscv_class_max_nregs (reg_class_t rclass, enum machine_mode mode)
   return 0;
 }
 
-/* Implement TARGET_PREFERRED_RELOAD_CLASS.  */
-
-static reg_class_t
-riscv_preferred_reload_class (rtx x ATTRIBUTE_UNUSED, reg_class_t rclass)
-{
-  return reg_class_subset_p (FP_REGS, rclass) ? FP_REGS :
-	 reg_class_subset_p (GR_REGS, rclass) ? GR_REGS :
-	 rclass;
-}
-
 /* Implement TARGET_MEMORY_MOVE_COST.  */
 
 static int
@@ -4031,9 +4021,6 @@ riscv_cannot_copy_insn_p (rtx_insn *insn)
 #undef TARGET_ADDRESS_COST
 #define TARGET_ADDRESS_COST riscv_address_cost
 
-#undef  TARGET_PREFERRED_RELOAD_CLASS
-#define TARGET_PREFERRED_RELOAD_CLASS riscv_preferred_reload_class
-
 #undef TARGET_ASM_FILE_START
 #define TARGET_ASM_FILE_START riscv_file_start
 #undef TARGET_ASM_FILE_START_FILE_DIRECTIVE