[APInt] Don't initialize VAL to 0 in APInt constructors. Push it down to the...

[APInt] Don't initialize VAL to 0 in APInt constructors. Push it down to the initSlowCase and other init methods.

I'm not sure if zeroing VAL before writing pVal is really necessary, but at least one other place did it in code.

But by taking the store out of line, this reduces the opt binary by about 20k on my local x86-64 build.

llvm-svn: 298233