[PowerPC] Support constraint 'wi' in asm

[HLJ2009]

Summary

From the gcc manual, we can see that the specific limit of wi inline asm is “FP or VSX register to hold 64-bit integers for VSX insns or NO_REGS”. The link is https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/Machine-Constraints.html#Machine-Constraints.
If we use the following code to test

int foo(vector float __A)
{
  int res = 0;
  vector float vtmp;
  __asm__(
      "xxsldwi %x1,%x2,%x2,3;\n"
      "xscvspdp %x1,%x1;\n"
      "fctiw  %1,%1;\n"
      "mfvsrd  %0,%x1;\n"
      : "=r" (res),
        "=&wi" (vtmp)
      : "wa" (__A)
      : );
  return (res);
}

We found that it would produce the following error
asm_wi.c:12:2: error: invalid output constraint '=&wi' in asm
"=&wi" (vtmp)
^
1 error generated.
We should accept this constraint.

[HLJ2009]

gentle pinging

[jsji]

This LGTM. Thanks for fixing.

lib/Basic/Targets/PPC.h

case 'v': // Altivec vector register
       break;
     case 'w':
       switch (Name[1]) {
       case 'd': // VSX vector register to hold vector double data
       case 'f': // VSX vector register to hold vector float data
       case 's': // VSX vector register to hold scalar float data
       case 'a': // Any VSX register
       case 'c': // An individual CR bit
+      case 'i': // FP or VSX register to hold 64-bit integers data
         break;
       default:
         return false;
       }
       Info.setAllowsRegister();
       Name++; // Skip over 'w'.
       break;
     case 'h': // `MQ', `CTR', or `LINK' register