[NFC][PowerPC] Update builtins-ppc-altivec.c to be run under `-faltivec-src-compat=mixed`

[amyk]

Summary

This patch adds the -faltivec-src-compat=mixed option to the builtins-ppc-altivec.c test.

Currently, the default for -faltivec-src-compat is mixed. The reason we explicitly specify mixed
to the RUN lines of this test is because eventually, the default will set to xl.

Having the default as xl changes the CHECKs of this test slightly, as it reorders some of the
vector bool and vector pixel CHECKs (since under the xl option, vector bool and vector pixel
are treated in the same way as other vector scalars). Explicitly specifying mixed ensures that
we are testing pre-existing Clang behaviour.

[nemanjai]

LGTM. Please don't forget to mark this as NFC in the title.

clang/test/CodeGen/builtins-ppc-altivec.c

 // REQUIRES: powerpc-registered-target
 // RUN: %clang_cc1 -target-feature +altivec -triple powerpc-unknown-unknown -emit-llvm %s \
-// RUN:            -flax-vector-conversions=none -o - | FileCheck %s
+// RUN:            -flax-vector-conversions=none  -faltivec-src-compat=mixed \
+// RUN:            -o - | FileCheck %s
 // RUN: %clang_cc1 -target-feature +altivec -triple powerpcle-unknown-unknown -emit-llvm %s \
-// RUN:            -flax-vector-conversions=none -o - | FileCheck %s -check-prefix=CHECK-LE
+// RUN:            -flax-vector-conversions=none  -faltivec-src-compat=mixed \
+// RUN:            -o - | FileCheck %s -check-prefix=CHECK-LE
 // RUN: %clang_cc1 -target-feature +altivec -triple powerpc64-unknown-unknown -emit-llvm %s \
-// RUN:            -flax-vector-conversions=none -o - | FileCheck %s
+// RUN:            -flax-vector-conversions=none  -faltivec-src-compat=mixed \
+// RUN:            -o - | FileCheck %s
 // RUN: %clang_cc1 -target-feature +altivec -triple powerpc64le-unknown-unknown -emit-llvm %s \
-// RUN:            -flax-vector-conversions=none -o - | FileCheck %s -check-prefix=CHECK-LE
+// RUN:            -flax-vector-conversions=none  -faltivec-src-compat=mixed \
+// RUN:            -o - | FileCheck %s -check-prefix=CHECK-LE
 // RUN: not %clang_cc1 -triple powerpc64le-unknown-unknown -emit-llvm %s \
 // RUN:            -ferror-limit 0 -DNO_ALTIVEC -o - 2>&1 \
 // RUN:            | FileCheck %s -check-prefix=CHECK-NOALTIVEC
 #ifndef NO_ALTIVEC
 #include <altivec.h>
 #endif
 
 vector bool char vbc = { 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0 };