x86-64: tighten convert-load-reloc checking

Even if the assembler avoids using relaxable relocations for
inapplicable insns, such relocations can still appear for other reasons.
Be more thorough in the opcode checking we do, to avoid bogusly altering
other insns.

Furthermore correct an opcode mask (even if with the added condition
that's now fully benign).

bfd/elf64-x86-64.c

@@ -2222,13 +2222,15 @@ elf_x86_64_convert_load_reloc (bfd *abfd,
 	      modrm = 0xc0 | (modrm & 0x38) >> 3;
 	      opcode = 0xf7;
 	    }
-	  else
+	  else if ((opcode | 0x38) == 0x3b)
 	    {
 	      /* Convert "binop foo@GOTPCREL(%rip), %reg" to
 		 "binop $foo, %reg".  */
-	      modrm = 0xc0 | (modrm & 0x38) >> 3 | (opcode & 0x3c);
+	      modrm = 0xc0 | ((modrm & 0x38) >> 3) | (opcode & 0x38);
 	      opcode = 0x81;
 	    }
+	  else
+	    return true;
 
 	  /* Use R_X86_64_32 with 32-bit operand to avoid relocation
 	     overflow when sign-extending imm32 to imm64.  */

ld/testsuite/ld-x86-64/load4.d

@@ -0,0 +1,13 @@
+#as: --64 -mrelax-relocations=yes
+#ld: -melf_x86_64 -z max-page-size=0x200000 -z noseparate-code
+#objdump: -dw
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+4000b0 <_start>:
+[ 	]*[a-f0-9]+:	12 05 ([0-9a-f]{2} ){4} *	adc    0x[a-f0-9]+\(%rip\),%al        # 6000c8 <.*>
+[ 	]*[a-f0-9]+:	44 84 3d ([0-9a-f]{2} ){4} *	test   %r15b,0x[a-f0-9]+\(%rip\)        # 6000c8 <.*>
+[ 	]*[a-f0-9]+:	48 87 05 ([0-9a-f]{2} ){4} *	xchg   %rax,0x[a-f0-9]+\(%rip\)        # 6000c8 <.*>
+#pass

ld/testsuite/ld-x86-64/load4.s

@@ -0,0 +1,23 @@
+	.data
+	.type	bar, @object
+bar:
+	.byte	1
+	.size	bar, .-bar
+	.globl	foo
+	.type	foo, @object
+foo:
+	.byte	1
+	.size	foo, .-foo
+	.text
+	.globl	_start
+	.type	_start, @function
+_start:
+	# Other insns must not be accidentally transformed.
+	adc	1f(%rip), %al
+1:	.reloc .-4, R_X86_64_GOTPCRELX, bar-4
+	test	%r15b, 1f(%rip)
+1:	.reloc .-4, R_X86_64_REX_GOTPCRELX, bar-4
+	xchg	1f(%rip), %rax
+1:	.reloc .-4, R_X86_64_REX_GOTPCRELX, bar-4
+
+	.size	_start, .-_start

ld/testsuite/ld-x86-64/x86-64.exp

@@ -653,6 +653,7 @@ run_dump_test "apx-load1d"
 run_dump_test "load2"
 run_dump_test "load3a"
 run_dump_test "load3b"
+run_dump_test "load4"
 run_dump_test "call1a"
 run_dump_test "call1b"
 run_dump_test "call1c"