[Doc] Document "Splat" in the lexicon

[chenwj]

Summary

As suggested on [1], adding Splat into LLVM Lexicon to benefit others.

[1] http://lists.llvm.org/pipermail/llvm-dev/2017-May/112757.html

[spatel]

LGTM. A couple of potential changes, but I leave that to you if you think it's better:

1. A shorter statement:

Splat refers to a vector of identical scalar elements.

2. We could add a sentence for the historical background:

The term is based on the PowerPC Altivec instructions that provided this functionality in hardware. For example, "vsplth" and the corresponding software intrinsic "vec_splat()".

[aemerson]

Splat is a synonym for broadcast as well, probably worth adding a mention.

[chenwj]

@spatel @aemerson Done. If everything looks fine, please help me commit it as I have no commit access, thanks.

[spatel]

How about we add another hardware example and group those together:

Splat refers to a vector of identical scalar elements.

The term is based on the PowerPC Altivec instructions that provided
this functionality in hardware. For example, "vsplth" and the corresponding
software intrinsic "vec_splat()". Other hardware names for this action may be
"duplicate" (ARM) or "broadcast" (x86).

[chenwj]

Add more example.

[chenwj]

In D32964#750942, @spatel wrote:

How about we add another hardware example and group those together:

Done. I think we already cover all major hardwares. ;-)

docs/Lexicon.rst

 
 **SSA**
     Static Single Assignment
 
 **Stack Map**
     In garbage collection, metadata emitted by the code generator which
     identifies `roots`_ within the stack frame of an executing function.
 
+**Splat**
+    Splat means to take a single value and build a vector out of identical
+    copies of that value.
+
 T
 -
 
 **TBAA**
     Type-Based Alias Analysis