message: Fix syntax of 'call' and 'splat' ops
  fix missing return value syntax on call / splat ops
  reflow cond_br / store op syntax
  Closes
  COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/161 from bondhugula:patch-1
diff: @@ -38,8 +38,7 @@ The MLIR branch operation is not allowed to target the entry block for a region.
  Syntax:
  ``` {.ebnf}
  -operation ::=
  - `cond_br` ssa-use `,` successor `,` successor
  +operation ::= `cond_br` ssa-use `,` successor `,` successor
  ```
  The `cond_br` terminator operation represents a conditional branch on a boolean
  @@ -85,7 +84,8 @@ single function to return.
  Syntax:
  ``` {.ebnf}
  -operation ::= `call` symbol-ref-id `(` ssa-use-list? `)` `:` function-type
  +operation ::=
  + (ssa-id `=`)? `call` symbol-ref-id `(` ssa-use-list? `)` `:` function-type
  ```
  The `call` operation represents a direct call to a function. The operands and
  @@ -357,7 +357,7 @@ in these contexts.
  Syntax:
  ``` {.ebnf}
  -operation ::= `splat` ssa-use `:` ( vector-type | tensor-type )
  +operation ::= ssa-id `=` `splat` ssa-use `:` ( vector-type | tensor-type )
  ```
  Broadcast the operand to all elements of the result vector or tensor. The
  @@ -383,8 +383,7 @@ sizes of the two dynamic dimensions. %m = "foo"() : () -> (index) %n = "bar"() :
  Syntax:
  ``` {.ebnf}
  -operation ::= `store` ssa-use `,` ssa-use
  - `[` ssa-use-list `]` `:` memref-type
  +operation ::= `store` ssa-use `,` ssa-use `[` ssa-use-list `]` `:` memref-type
  ```
  Store value to memref location given by indices. The value stored should have
  