Redo rep string expansion to avoid the instruction fetch on subsequent iterations

This is related to #4915 where we may remove the instr fetches for subsequent rep string iterations, currently marked as special "non-fetched" instructions, from the offline drcachesim trace post-processing. Here though the goal is remove the instruction fetch for all clients by changing the expansion.

Today the single-block-external-loop expansion results in a string operation instruction in each iteration:

$ bin64/drrun -c api/bin/libmemtrace_simple.so -- suite/tests/bin/allasm_repstr && cat `ls -1td api/bin/*.log|head -1`
Format: <data address>: <data size>, <(r)ead/(w)rite/opcode>
0x401018:  2, rep movs
0x40200e:  1, r
0x402000:  1, w
0x401018:  2, rep movs
0x40200f:  1, r
0x402001:  1, w
0x401018:  2, rep movs
0x402010:  1, r
0x402002:  1, w
0x401018:  2, rep movs
0x402011:  1, r
0x402003:  1, w
0x401018:  2, rep movs
0x402012:  1, r
0x402004:  1, w

There are 3 ways we could remove it (aside from an offline post-processing model as in #4915):

A) Use runtime state:

if TLS.flag == 0
  record instr fetch
  set TLS.flag to 1
<today's expansion but without emulated instr fetch>
if loop is going to be not taken (check ecx)
  set TLS.flag to 0
loop

This is expensive.

B) Unroll the 1st iter in the expansion. This adds a bunch more branches and we'll want to try to ensure it won't mess up drreg or other linear-flow libraries.

C) Unroll the 1st iter across 2 blocks: 1st iter block plus rest-of-loop block. This may be harder than B due to needing unique PC entries.

Probably B would be the way to go.