[asan] extend asan-coverage (still experimental).
 - add a mode for collecting per-block coverage (-asan-coverage=2).
   So far the implementation is naive (all blocks are instrumented),
   the performance overhead on top of asan could be as high as 30%.
 - Make sure the one-time calls to __sanitizer_cov are moved to function buttom,
   which in turn required to copy the original debug info into the call insn.

Here is the performance data on SPEC 2006
(train data, comparing asan with asan-coverage={0,1,2}):

                             asan+cov0     asan+cov1      diff 0-1    asan+cov2       diff 0-2      diff 1-2
       400.perlbench,        65.60,        65.80,         1.00,        76.20,         1.16,         1.16
           401.bzip2,        65.10,        65.50,         1.01,        75.90,         1.17,         1.16
             403.gcc,         1.64,         1.69,         1.03,         2.04,         1.24,         1.21
             429.mcf,        21.90,        22.60,         1.03,        23.20,         1.06,         1.03
           445.gobmk,       166.00,       169.00,         1.02,       205.00,         1.23,         1.21
           456.hmmer,        88.30,        87.90,         1.00,        91.00,         1.03,         1.04
           458.sjeng,       210.00,       222.00,         1.06,       258.00,         1.23,         1.16
      462.libquantum,         1.73,         1.75,         1.01,         2.11,         1.22,         1.21
         464.h264ref,       147.00,       152.00,         1.03,       160.00,         1.09,         1.05
         471.omnetpp,       115.00,       116.00,         1.01,       140.00,         1.22,         1.21
           473.astar,       133.00,       131.00,         0.98,       142.00,         1.07,         1.08
       483.xalancbmk,       118.00,       120.00,         1.02,       154.00,         1.31,         1.28
            433.milc,        19.80,        20.00,         1.01,        20.10,         1.02,         1.01
            444.namd,        16.20,        16.20,         1.00,        17.60,         1.09,         1.09
          447.dealII,        41.80,        42.20,         1.01,        43.50,         1.04,         1.03
          450.soplex,         7.51,         7.82,         1.04,         8.25,         1.10,         1.05
          453.povray,        14.00,        14.40,         1.03,        15.80,         1.13,         1.10
             470.lbm,        33.30,        34.10,         1.02,        34.10,         1.02,         1.00
         482.sphinx3,        12.40,        12.30,         0.99,        13.00,         1.05,         1.06