[NFC][CodeGen][X86][AArch64] Tests for bit extract (pat. b) with trunc (PR36419)

@bextr64_32_b1 is extracted from hotpath of real-world code
(RawSpeed BitStream<>::peekBitsNoFill()) after `clang -O3`.

@bextr64_32_b2/@bextr64_32_b0 is the same pattern,
but with trunc done last, showing how i think it can be handled:
https://rise4fun.com/Alive/K4B
https://rise4fun.com/Alive/qC9

It is possible that middle-end should do some of this, too.

https://bugs.llvm.org/show_bug.cgi?id=36419

llvm-svn: 349998