[LangRef] Update description of Constant Expressions

[LangRef] Update description of Constant Expressions

Summary:
When describing trunc/zext/sext/ptrtoint/inttoptr in the chapter
about Constant Expressions we now simply refer to the Instruction
Reference. As far as I know there are no difference when it comes
to the semantics and the argument constraints. The only difference
is that the syntax is slighly different for the constant expressions,
regarding the use of parenthesis in constant expressions.
Referring to the Instruction Reference is the same solution as
already used for several other operations, such as bitcast.

The main goal was to add information that vector types are allowed
also in trunc/zext/sext/ptrtoint/inttoptr constant expressions.
That was not explicitly mentioned earlier, and resulted in some
questions in the review of https://reviews.llvm.org/D38546

Reviewers: efriedma, majnemer

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39165

llvm-svn: 316429