[APInt] In sext single word case, use SignExtend64 and let the APInt...

[APInt] In sext single word case, use SignExtend64 and let the APInt constructor mask off any excess bits.

The current code is trying to be clever with shifts to avoid needing to clear unused bits. But it looks like the compiler is unable to optimize out the unused bit handling in the APInt constructor. Given this its better to just use SignExtend64 and have more readable code.

llvm-svn: 301133