[update_llc_test_checks.py] Fix MIPS ASM regex for functions with EH

On MIPS, functions with exception handling code emits an additional
temporary label at the start of the function (due to UseAssignmentForEHBegin):

    _Z8do_catchv:                           # @_Z8do_catchv
    .Ltmp3:
    .set .Lfunc_begin0, .Ltmp3
    .cfi_startproc
    .cfi_personality 128, DW.ref.__gxx_personality_v0
    .cfi_lsda 0, .Lexception0
    .frame	$c11,48,$c17
    .mask 	0x00000000,0
    .fmask	0x00000000,0
    .set	noreorder
    .set	nomacro
    .set	noat
    # %bb.0:                                # %entry

The `[^:]*` regex was terminating the search after .Ltmp<N>: and therefore
not detecting functions with exception handling.

Reviewed By: atanasyan, MaskRay

Differential Revision: https://reviews.llvm.org/D100027