[FileCheck]Remove assertions that prevent matching an empty string at file...

[FileCheck]Remove assertions that prevent matching an empty string at file start before CHECK-NEXT/SAME

This patch removes two assertions that were preventing writing of a test
that checked an empty line followed by some text. For example:

CHECK: {{^$}}
CHECK-NEXT: foo()

The assertion was because the current location the CHECK-NEXT was
scanning from was the start of the buffer. A similar issue occurred with
CHECK-SAME. These assertions don't protect against anything, as there is
already an error check that checks that CHECK-NEXT/EMPTY/SAME don't
appear first in the checks, and the following code works fine if the
pointer is at the start of the input.

Reviewed by: probinson, thopre, jdenny
Differential Revision: https://reviews.llvm.org/D58784

llvm-svn: 355928