[NFC][PowerPC] Use -check-prefixes to simplify the check in code-align.ll

Summary:
When checking the same output, we can use the `-check-prefixes` to simplify the check.
For example, if we want to check below output.
```
; GENERIC-LABEL: .globl  foo
; BASIC-LABEL: .globl  foo
; PWR-LABEL: .globl  foo
; GENERIC: .p2align  2
; BASIC: .p2align  4
; PWR: .p2align  4
; GENERIC: @foo
; BASIC: @foo
; PWR: @foo

```
If we use `-check-prefixes`
```
... -check-prefixes=CHECK,GENERAL
... -check-prefixes=CHECK,BASIC
... -check-prefixes=CHECK,PWR
```
Above check can be simplify to:
```
; CHECK-LABEL: .globl  foo
; GENERIC: .p2align  2
; BASIC: .p2align  4
; PWR: .p2align  4
; CHECK: @foo
```

Reviewed By: hfinkel
Differential Revision: https://reviews.llvm.org/D61227

llvm-svn: 359533