rs6000: Make cr5 allocatable

A comment in rs6000.h says "cr5 is not supposed to be used".  I checked
all ABIs, going as far back as PowerOpen (1994), and found no mention
of this.

Also document cr6 is used by some vector instructions.

From-SVN: r211811

gcc/ChangeLog

+2014-06-18  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	* config/rs6000/rs6000.h (FIXED_REGISTERS): Update comment.
+	Remove cr5.
+	(REG_ALLOC_ORDER): Update comment.  Move cr5 earlier.
+
 2014-06-18  Kaz Kojima  <kkojima@gcc.gnu.org>
 
 	PR target/61550

gcc/config/rs6000/rs6000.h

@@ -978,8 +978,6 @@ enum data_align { align_abi, align_opt, align_both };
    On RS/6000, r1 is used for the stack.  On Darwin, r2 is available
    as a local register; for all other OS's r2 is the TOC pointer.
 
-   cr5 is not supposed to be used.
-
    On System V implementations, r13 is fixed and not available for use.  */
 
 #define FIXED_REGISTERS  \
@@ -987,7 +985,7 @@ enum data_align { align_abi, align_opt, align_both };
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \
-   0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1,	   \
+   0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1,	   \
    /* AltiVec registers.  */			   \
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \
@@ -1048,7 +1046,8 @@ enum data_align { align_abi, align_opt, align_both };
 	fp13 - fp2	(not saved; incoming fp arg registers)
 	fp1		(not saved; return value)
 	fp31 - fp14	(saved; order given to save least number)
-	cr7, cr6	(not saved or special)
+	cr7, cr5	(not saved or special)
+	cr6		(not saved, but used for vector operations)
 	cr1		(not saved, but used for FP operations)
 	cr0		(not saved, but used for arithmetic operations)
 	cr4, cr3, cr2	(saved)
@@ -1061,7 +1060,7 @@ enum data_align { align_abi, align_opt, align_both };
 	r12		(not saved; if used for DImode or DFmode would use r13)
 	ctr		(not saved; when we have the choice ctr is better)
 	lr		(saved)
-	cr5, r1, r2, ap, ca (fixed)
+	r1, r2, ap, ca	(fixed)
 	v0 - v1		(not saved or used for anything)
 	v13 - v3	(not saved; incoming vector arg registers)
 	v2		(not saved; incoming vector arg reg; return value)
@@ -1099,14 +1098,14 @@ enum data_align { align_abi, align_opt, align_both };
    33,								\
    63, 62, 61, 60, 59, 58, 57, 56, 55, 54, 53, 52, 51,		\
    50, 49, 48, 47, 46,						\
-   75, 74, 69, 68, 72, 71, 70,					\
+   75, 73, 74, 69, 68, 72, 71, 70,				\
    MAYBE_R2_AVAILABLE						\
    9, 10, 8, 7, 6, 5, 4,					\
    3, EARLY_R12 11, 0,						\
    31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19,		\
    18, 17, 16, 15, 14, 13, LATE_R12				\
    66, 65,							\
-   73, 1, MAYBE_R2_FIXED 67, 76,				\
+   1, MAYBE_R2_FIXED 67, 76,					\
    /* AltiVec registers.  */					\
    77, 78,							\
    90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80,			\