[WebAssembly] Place 'try' and 'catch' correctly wrt EH_LABELs

Summary:
After instruction selection phase, possibly-throwing calls, which were
previously invoke, are wrapped in `EH_LABEL` instructions. For example:
```
  EH_LABEL <mcsymbol .Ltmp0>
  CALL_VOID @foo ...
  EH_LABEL <mcsymbol .Ltmp1>
```

`EH_LABEL` is placed also in the beginning of EH pads:
```
bb.1 (landing-pad):
  EH_LABEL <mcsymbol .Ltmp2>
  ...
```

And we'd like to maintian this relationship, so when we place a `try`,
```
  TRY ...
  EH_LABEL <mcsymbol .Ltmp0>
  CALL_VOID @foo ...
  EH_LABEL <mcsymbol .Ltmp1>
```

When we place a `catch`,
```
bb.1 (landing-pad):
  EH_LABEL <mcsymbol .Ltmp2>
  %0:except_ref = CATCH ...
  ...
```

Previously we didn't treat EH_LABELs specially, so `try` was placed
right before a call, and `catch` was placed in the beginning of an EH
pad.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58914

llvm-svn: 355996