Revert "s/grep/FileCheck/ in some tests"

This reverts commit 8b75e6bc35fb3f9c1e788dbd05084c0f4a60a0f3.

The FileCheck tests are not equivalent:

test/CodeGen/X86/tailcall-structret.ll:6:10: error: expected string not found in input
; CHECK: jmp init
         ^
<stdin>:1:2: note: scanning from here
 .section __TEXT,__text,regular,pure_instructions
 ^
<stdin>:13:2: note: possible intended match here
 jmp _init ## TAILCALL
 ^