X86: Rework inline asm integer register specification.

This is a new version of http://reviews.llvm.org/D10260.

It turned out that when you specify an integer register in inline asm on
x86 you get the register of the required type size back. That means that
X86TargetLowering::getRegForInlineAsmConstraint() has to accept any of
the integer registers and adapt its size to the given target size which
may be any 8/16/32/64 bit sized type. Surprisingly that means given a
constraint of "{ax}" and a type of MVT::F32 we need to return X86::EAX.

This change makes this face explicit, the previous code seemed like
working by accident because there it never returned an error once a
register was found. On the other hand this rewrite allows to actually
return errors for invalid situations like requesting an integer register
for an i128 type.

Related to rdar://21042280

Differential Revision: http://reviews.llvm.org/D10813

llvm-svn: 241002